netfilter: fix wrong arithmetics regarding NFT_REJECT_ICMPX_MAX

NFT_REJECT_ICMPX_MAX should be __NFT_REJECT_ICMPX_MAX - 1.

nft_reject_icmp_code() and nft_reject_icmpv6_code() are called from the
packet path, so BUG_ON in case we try to access an unknown abstracted
ICMP code. This should not happen since we already validate this from
nft_reject_{inet,bridge}_init().

Fixes: 51b0a5d8 ("netfilter: nft_reject: introduce icmp code abstraction for inet and bridge")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>