net/sched: act_csum: compute crc32c on SCTP packets

modify act_csum to compute crc32c on IPv4/IPv6 packets having SCTP in
their payload, and extend UAPI definitions accordingly.

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>