netfilter: x_tables: fix compat match/target pad out-of-bound write

commit b29c457a

 upstream.

xt_compat_match/target_from_user doesn't check that zeroing the area
to start of next rule won't write past end of allocated ruleset blob.

Remove this code and zero the entire blob beforehand.

Reported-by:  <syzbot+cfc0247ac173f597aaaa@syzkaller.appspotmail.com>
Reported-by: Andy Nguyen <theflow@google.com>
Fixes: 9fa492cd

 ("[NETFILTER]: x_tables: simplify compat API")
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>