nfsd4: return "real" sequence id in confirmed case

The client should ignore the returned sequence_id in the case where the
CONFIRMED flag is set on an exchange_id reply--and in the unconfirmed
case "1" is always the right response.  So it shouldn't actually matter
what we return here.

We could continue returning 1 just to catch clients ignoring the spec
here, but I'd rather be generous.  Other things equal, returning the
existing sequence_id seems more informative.

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