evaluate: handle payload matching split in two bytes

evaluate: handle payload matching split in two bytes

When the bits are split between two bytes and the payload field is
smaller than one byte, we need to extend the expression length on both
sides (payload and constant) of the relational expression.

The existing trimming from the delinerization step handles the listing
for us, so no changes on that front.

This patch allows us to match the IPv6 DSCP field which falls into the
case that is described above.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>