[NETFILTER]: bridge: fix missing link layer headers on outgoing routed packets

As reported by Damien Thebault, the double POSTROUTING hook invocation
fix caused outgoing packets routed between two bridges to appear without
a link-layer header. The reason for this is that we're skipping the
br_nf_post_routing hook for routed packets now and don't save the
original link layer header, but nevertheless tries to restore it on
output, causing corruption.

The root cause for this is that skb->nf_bridge has no clearly defined
lifetime and is used to indicate all kind of things, but that is
quite complicated to fix. For now simply don't touch these packets
and handle them like packets from any other device.

Tested-by: Damien Thebault <damien.thebault@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>