net: WARN if skb_checksum_help() is called on skb requiring segmentation

skb_checksum_help() has never done anything useful with skbs that
require segmentation.  Setting skb->ip_summed = CHECKSUM_NONE makes
them invalid and provokes a later WARNing in skb_gso_segment().

Passing such an skb to skb_checksum_help() indicates a bug, so we
should warn about it immediately.  Move the warning from
skb_gso_segment() into a shared function, and add gso_type and
gso_size to it.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>