netfilter: compat: prepare xt_compat_init_offsets to return errors

commit 9782a11efc072faaf91d4aa60e9d23553f918029 upstream.

should have no impact, function still always returns 0.
This patch is only to ease review.

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