nfsd4: hash lockowners to simplify RELEASE_LOCKOWNER

Hash lockowners on just the owner string rather than on (owner, inode).
This makes the owner-string lookup needed for RELEASE_LOCKOWNER simpler
(currently it's doing at a linear search through the entire hash
table!).  That may come at the expense of making (owner, inode) lookups
more expensive if a client reuses the same lockowner across multiple
files.  We might add a separate lookup for that.

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