netfilter: conntrack: validate SCTP crc32c in PREROUTING

implement sctp_error to let nf_conntrack_in validate crc32c on the packet
transport header. Assign skb->ip_summed to CHECKSUM_UNNECESSARY and return
NF_ACCEPT in case of successful validation; otherwise, return -NF_ACCEPT to
let netfilter skip connection tracking, like other protocols do.

Besides preventing corrupted packets from matching conntrack entries, this
fixes functionality of REJECT target: it was not generating any ICMP upon
reception of SCTP packets, because it was computing RFC 1624 checksum on
the packet and systematically mismatching crc32c in the SCTP header.

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>