bridge : Sanitize skb before it enters the IP stack

Related dicussion here : http://lkml.org/lkml/2010/9/3/16

Introduce a function br_parse_ip_options that will audit the
skb and possibly refill IP options before a packet enters the
IP stack. If no options are present, the function will zero out
the skb cb area so that it is not misinterpreted as options by some
unsuspecting IP layer routine. If packet consistency fails, drop it.

Signed-off-by: Bandan Das <bandan.das@stratus.com>
Signed-off-by: David S. Miller <davem@davemloft.net>