nfsd: clean up nfsd4_release_lockowner

Now that we know that we won't have several lockowners with the same,
owner->data, we can simplify nfsd4_release_lockowner and get rid of
the lo_list in the process.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>