net: core: Use skb_is_gso() in skb_checksum_help()

No functional changes, just minor refactoring.

Signed-off-by: Yi Li <yili@winhong.com>
Link: https://lore.kernel.org/r/20201027055904.2683444-1-yili@winhong.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>