nfsd4: do not throw away 4.1 lock state on last unlock

This reverts commit eb2099f3

 "nfsd4:
release lockowners on last unlock in 4.1 case".  Trond identified
language in rfc 5661 section 8.2.4 which forbids this behavior:

	Stateids associated with byte-range locks are an exception.
	They remain valid even if a LOCKU frees all remaining locks, so
	long as the open file with which they are associated remains
	open, unless the client frees the stateids via the FREE_STATEID
	operation.

And bakeathon 2013 testing found a 4.1 freebsd client was getting an
incorrect BAD_STATEID return from a FREE_STATEID in the above situation
and then failing.

The spec language honestly was probably a mistake but at this point with
implementations already following it we're probably stuck with that.

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