netfilter: ebtables: also count base chain policies

commit 3b48300d5cc7c7bed63fddb006c4046549ed4aec upstream.

ebtables doesn't include the base chain policies in the rule count,
so we need to add them manually when we call into the x_tables core
to allocate space for the comapt offset table.

This lead syzbot to trigger:
WARNING: CPU: 1 PID: 9012 at net/netfilter/x_tables.c:649
xt_compat_add_offset.cold+0x11/0x36 net/netfilter/x_tables.c:649

Reported-by:  <syzbot+276ddebab3382bbf72db@syzkaller.appspotmail.com>
Fixes: 2035f3ff8eaa ("netfilter: ebtables: compat: un-break 32bit setsockopt when no rules are present")
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>