openvswitch: more accurate checksumming in queue_userspace_packet()

if skb carries an SCTP packet and ip_summed is CHECKSUM_PARTIAL, it needs
CRC32c in place of Internet Checksum: use skb_csum_hwoffload_help to avoid
corrupting such packets while queueing them towards userspace.

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