net: igb: use skb_csum_is_sctp instead of protocol check

mainline inclusion
from mainline-v5.12-rc1-dontuse
commit 8bcf0203
category: feature
bugzilla: 173966
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=8bcf02035bd5ab5f22110d16a1aaee1794aa8d3c



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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>
Reviewed-by: Yongxin Li <liyongxin1@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>