netfilter: nft_reject_inet: fix unintended fall-through in switch-statatement

For IPv4 packets, we call both IPv4 and IPv6 reject.

Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>