nfsd4: release lockowners on last unlock in 4.1 case

In the 4.1 case we're supposed to release lockowners as soon as they're
no longer used.

It would probably be more efficient to reference count them, but that's
slightly fiddly due to the need to have callbacks from locks.c to take
into account lock merging and splitting.

For most cases just scanning the inode's lock list on unlock for
matching locks will be sufficient.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>