Check for a NULL dsaddr in filelayout_free_lseg() before calling
nfs4_fl_put_deviceid().  This fixes the following oops:

[ 1967.645207] BUG: unable to handle kernel NULL pointer dereference at 0000000000000030
[ 1967.646010] IP: [<ffffffffc06d6aea>] nfs4_put_deviceid_node+0xa/0x90 [nfsv4]
[ 1967.646010] PGD c08bc067 PUD 915d3067 PMD 0
[ 1967.753036] Oops: 0000 [#1] SMP
[ 1967.753036] Modules linked in: nfs_layout_nfsv41_files ext4 mbcache jbd2 loop rpcsec_gss_krb5 nfsv4 dns_resolver nfs fscache amd64_edac_mod ipmi_ssif edac_mce_amd edac_core kvm_amd sg kvm ipmi_si ipmi_devintf irqbypass pcspkr k8temp ipmi_msghandler i2c_piix4 shpchp nfsd auth_rpcgss nfs_acl lockd grace sunrpc ip_tables xfs libcrc32c sd_mod crc_t10dif crct10dif_generic crct10dif_common amdkfd amd_iommu_v2 radeon i2c_algo_bit drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops mptsas ttm scsi_transport_sas mptscsih drm mptbase serio_raw i2c_core bnx2 dm_mirror dm_region_hash dm_log dm_mod
[ 1967.790031] CPU: 2 PID: 1370 Comm: ls Not tainted 3.10.0-709.el7.test.bz1463784.x86_64 #1
[ 1967.790031] Hardware name: IBM BladeCenter LS21 -[7971AC1]-/Server Blade, BIOS -[BAE155AUS-1.10]- 06/03/2009
[ 1967.790031] task: ffff8800c42a3f40 ti: ffff8800c4064000 task.ti: ffff8800c4064000
[ 1967.790031] RIP: 0010:[<ffffffffc06d6aea>]  [<ffffffffc06d6aea>] nfs4_put_deviceid_node+0xa/0x90 [nfsv4]
[ 1967.790031] RSP: 0000:ffff8800c4067978  EFLAGS: 00010246
[ 1967.790031] RAX: ffffffffc062f000 RBX: ffff8801d468a540 RCX: dead000000000200
[ 1967.790031] RDX: ffff8800c40679f8 RSI: ffff8800c4067a0c RDI: 0000000000000000
[ 1967.790031] RBP: ffff8800c4067980 R08: ffff8801d468a540 R09: 0000000000000000
[ 1967.790031] R10: 0000000000000000 R11: ffffffffffffffff R12: ffff8801d468a540
[ 1967.790031] R13: ffff8800c40679f8 R14: ffff8801d5645300 R15: ffff880126f15ff0
[ 1967.790031] FS:  00007f11053c9800(0000) GS:ffff88012bd00000(0000) knlGS:0000000000000000
[ 1967.790031] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 1967.790031] CR2: 0000000000000030 CR3: 0000000094b55000 CR4: 00000000000007e0
[ 1967.790031] Stack:
[ 1967.790031]  ffff8801d468a540 ffff8800c4067990 ffffffffc062d2fe ffff8800c40679b0
[ 1967.790031]  ffffffffc062b5b4 ffff8800c40679f8 ffff8801d468a540 ffff8800c40679d8
[ 1967.790031]  ffffffffc06d39af ffff8800c40679f8 ffff880126f16078 0000000000000001
[ 1967.790031] Call Trace:
[ 1967.790031]  [<ffffffffc062d2fe>] nfs4_fl_put_deviceid+0xe/0x10 [nfs_layout_nfsv41_files]
[ 1967.790031]  [<ffffffffc062b5b4>] filelayout_free_lseg+0x24/0x90 [nfs_layout_nfsv41_files]
[ 1967.790031]  [<ffffffffc06d39af>] pnfs_free_lseg_list+0x5f/0x80 [nfsv4]
[ 1967.790031]  [<ffffffffc06d5a67>] _pnfs_return_layout+0x157/0x270 [nfsv4]
[ 1967.790031]  [<ffffffffc06c17dd>] nfs4_evict_inode+0x4d/0x70 [nfsv4]
[ 1967.790031]  [<ffffffff8121de19>] evict+0xa9/0x180
[ 1967.790031]  [<ffffffff8121e729>] iput+0xf9/0x190
[ 1967.790031]  [<ffffffffc0652cea>] nfs_dentry_iput+0x3a/0x50 [nfs]
[ 1967.790031]  [<ffffffff8121ab4f>] shrink_dentry_list+0x20f/0x490
[ 1967.790031]  [<ffffffff8121b018>] d_invalidate+0xd8/0x150
[ 1967.790031]  [<ffffffffc065446b>] nfs_readdir_page_filler+0x40b/0x600 [nfs]
[ 1967.790031]  [<ffffffffc0654bbd>] nfs_readdir_xdr_to_array+0x20d/0x3b0 [nfs]
[ 1967.790031]  [<ffffffff811f3482>] ? __mem_cgroup_commit_charge+0xe2/0x2f0
[ 1967.790031]  [<ffffffff81183208>] ? __add_to_page_cache_locked+0x48/0x170
[ 1967.790031]  [<ffffffffc0654d60>] ? nfs_readdir_xdr_to_array+0x3b0/0x3b0 [nfs]
[ 1967.790031]  [<ffffffffc0654d82>] nfs_readdir_filler+0x22/0x90 [nfs]
[ 1967.790031]  [<ffffffff8118351f>] do_read_cache_page+0x7f/0x190
[ 1967.790031]  [<ffffffff81215d30>] ? fillonedir+0xe0/0xe0
[ 1967.790031]  [<ffffffff8118366c>] read_cache_page+0x1c/0x30
[ 1967.790031]  [<ffffffffc0654f9b>] nfs_readdir+0x1ab/0x6b0 [nfs]
[ 1967.790031]  [<ffffffffc06bd1c0>] ? nfs4_xdr_dec_layoutget+0x270/0x270 [nfsv4]
[ 1967.790031]  [<ffffffff81215d30>] ? fillonedir+0xe0/0xe0
[ 1967.790031]  [<ffffffff81215c20>] vfs_readdir+0xb0/0xe0
[ 1967.790031]  [<ffffffff81216045>] SyS_getdents+0x95/0x120
[ 1967.790031]  [<ffffffff816b9449>] system_call_fastpath+0x16/0x1b
[ 1967.790031] Code: 90 31 d2 48 89 d0 5d c3 85 f6 74 f5 8d 4e 01 89 f0 f0 0f b1 0f 39 f0 74 e2 89 c6 eb eb 0f 1f 40 00 66 66 66 66 90 55 48 89 e5 53 <48> 8b 47 30 48 89 fb a8 04 74 3b 8b 57 60 83 fa 02 74 19 8d 4a
[ 1967.790031] RIP  [<ffffffffc06d6aea>] nfs4_put_deviceid_node+0xa/0x90 [nfsv4]
[ 1967.790031]  RSP <ffff8800c4067978>
[ 1967.790031] CR2: 0000000000000030

Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Fixes: 1ebf9801

 ("NFS/filelayout: Fix racy setting of fl->dsaddr...")
Cc: stable@vger.kernel.org # v4.13+
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>