lockd: set fl_owner when unlocking files

Unlocking a POSIX lock on an inode with vfs_lock_file only works if
the owner matches. Ensure we set it in the request.

Cc: J. Bruce Fields <bfields@fieldses.org>
Fixes: 7f024fcd

 ("Keep read and write fds with each nlm_file")
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>