net: add inline function skb_csum_is_sctp

mainline inclusion
from mainline-v5.12-rc1-dontuse
commit fa821170
category: feature
bugzilla: 173966
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fa82117010430aff2ce86400f7328f55a31b48a6



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This patch is to define a inline function skb_csum_is_sctp(), and
also replace all places where it checks if it's a SCTP CSUM skb.
This function would be used later in many networking drivers in
the following patches.

Suggested-by: Alexander Duyck <alexander.duyck@gmail.com>
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>