nfsd4: lockt, release_lockowner should renew clients

Fix nfsd4_lockt and release_lockowner to lookup the referenced client,
so that it can renew it, or correctly return "expired", as appropriate.

Also share some code while we're here.

Reported-by: Frank Filz <ffilzlnx@us.ibm.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>