netfilter: xt_rateest: fix xt_rateest_mt_checkentry()

commit 4a5a5c73 (slightly better error reporting) added some
useless code in xt_rateest_mt_checkentry().

Fix this so that different error codes can really be returned.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>