payload: don't kill dependency for proto_th

payload: don't kill dependency for proto_th

proto_th carries no information about the proto number, we need to
preserve the L4 protocol expression unless we can be sure that

For example, if "meta l4proto 91 @th,0,16 0" is simplified to
"th sport 0", the information of protocol number is lost.

Based on initial patch from Xiao Liang.

Signed-off-by: Xiao Liang <shaw.leon@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>