nfsd: optimise the starting of zero threads when none are running.

Currently, if we ask to set then number of nfsd threads to zero when
there are none running, we set up all the sockets and register the
service, and then tear it all down again.
This is pointless.

So detect that case and exit promptly.
(also remove an assignment to 'error' which was never used.

Signed-off-by: NeilBrown <neilb@suse.de>
Acked-by: Jeff Layton <jlayton@redhat.com>