netfilter: ebtables: CONFIG_COMPAT: drop a bogus WARN_ON

It means userspace gave us a ruleset where there is some other
data after the ebtables target but before the beginning of the next rule.

Fixes: 81e675c2 ("netfilter: ebtables: add CONFIG_COMPAT support")
Reported-by:  <syzbot+659574e7bcc7f7eb4df7@syzkaller.appspotmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>