payload: Update the context only in equality relations

payload: Update the context only in equality relations

If we add this rule:

sudo nft add rule ip test input ip protocol != icmp

and we try to list the rules in the table test, nftables
show this error:

nft: src/payload.c:76: payload_expr_pctx_update: Assertion `expr->op == OP_EQ' failed.

This patch change the function payload_match_postprocess for updating
only the context in equality relations case.

Signed-off-by: Alvaro Neira Ayuso <alvaroneay@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>