nfsd4: hash closed stateid's like any other

Look up closed stateid's in the stateid hash like any other stateid
rather than searching the close lru.

This is simpler, and fixes a bug: currently we handle only the case of a
close that is the last close for a given stateowner, but not the case of
a close for a stateowner that still has active opens on other files.
Thus in a case like:

	open(owner, file1)
	open(owner, file2)
	close(owner, file2)
	close(owner, file2)

the final close won't be recognized as a retransmission.

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