nfsd4: add a separate (lockowner, inode) lookup

Address the possible performance regression mentioned in "nfsd4: hash
lockowners to simplify RELEASE_LOCKOWNER" by providing a separate
(lockowner, inode) hash.

Really, I doubt this matters much, but I think it's likely we'll change
these data structures here and I'd rather that the need for (owner,
inode) lookups be well-documented.

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