vfs_lock_file() expects the struct file_lock to be fully initialised by
the caller. Re-exported NFSv3 has been seen to Oops if the fl_file field
is NULL.

Fixes: aec15824 ("lockd: set fl_owner when unlocking files")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216582Signed-off-by: Chuck Lever <chuck.lever@oracle.com>