netfilter: x_tables: use correct integer types

Sparse complains because __be32 and u32 are mixed without
conversions.  Use the correct types, no code changes.

Furthermore, xt_DSCP generates a bit truncation warning:
"cast truncates bits from constant value (ffffff03 becomes 3)"

The truncation is fine (and wanted). Add a private definition and use that
instead.

objdiff shows no changes.

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