netfilter: ebtables: convert BUG_ONs to WARN_ONs

All of these conditions are not fatal and should have
been WARN_ONs from the get-go.

Convert them to WARN_ONs and bail out.

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