nfsd: open a file descriptor for fsync in nfs4 recovery

Instead of just looking up a path use do_filp_open to get us a file
structure for the nfs4 recovery directory.  This allows us to get
rid of the last non-standard vfs_fsync caller with a NULL file
pointer.

[AV: should be using fput(), not filp_close()]

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>