bridge: reduce size of input cb to 16 bytes

Reduce size of br_input_skb_cb from 24 to 16 bytes by
using bitfield for those values that can only be 0 or 1.

igmp is the igmp type value, so it needs to be at least u8.

Furthermore, the bridge currently relies on step-by-step initialization
of br_input_skb_cb fields as the skb passes through the stack.

Explicitly zero out the bridge input cb instead, this avoids having to
review/validate that no BR_INPUT_SKB_CB(skb)->foo test can see a
'random' value from previous protocol cb.

AFAICS all current fields are always set up before they are read again,
so this is not a bug fix.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>