net: igb: use skb_csum_is_sctp instead of protocol check

Using skb_csum_is_sctp is a easier way to validate it's a SCTP
CRC checksum offload packet, and there is no need to parse the
packet to check its proto field, especially when it's a UDP or
GRE encapped packet.

So this patch also makes igb support SCTP CRC checksum offload
for UDP and GRE encapped packets.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>