netfilter: x_tables: validate all offsets and sizes in a rule

Validate that all matches (if any) add up to the beginning of
the target and that each match covers at least the base structure size.

The compat path should be able to safely re-use the function
as the structures only differ in alignment; added a
BUILD_BUG_ON just in case we have an arch that adds padding as well.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>