NFS: Fix an ABBA spinlock issue in pnfs_update_layout()

We need to drop the inode spinlock while calling nfs4_select_rw_stateid(),
since nfs4_copy_delegation_stateid() could take the delegation lock.
Note that it is safe to do this, since all other calls to
pnfs_update_layout() for that inode will find themselves blocked by
the lock we hold on NFS_LAYOUT_FIRST_LAYOUTGET.

Fixes: fc51b1cf ("NFS: Beware when dereferencing the delegation cred")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>