netfilter: bridge: rework reject handling

bridge reject handling is not straightforward, there are many subtle
differences depending on configuration.

skb->dev is either the bridge port (PRE_ROUTING) or the bridge
itself (INPUT), so we need to use indev instead.

Also, checksum validation will only work reliably if we trim skb
according to the l3 header size.

While at it, add csum validation for ipv6 and skip existing tests
if skb was already checked e.g. by GRO.

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