[NETFILTER]: bridge netfilter: deal with martians correctly

The attached patch resolves an issue where a IP DNATed packet with a
martian source is forwarded while it's better to drop it. It also
resolves messages complaining about ip forwarding being disabled while
it's actually enabled. Thanks to lepton <ytht.net@gmail.com> for
reporting this problem.

This is probably a candidate for the -stable release.

Signed-off-by: Bart De Schuymer <bdschuym@pandora.be>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>