bridge: Pull ip header into skb->data before looking into ip header.

[ Upstream commit 6caab7b0544e83e6c160b5e80f5a4a7dd69545c7 ]

If lower layer driver leaves the ip header in the skb fragment, it needs to
be first pulled into skb->data before inspecting ip header length or ip version
number.

Signed-off-by: Sarveshwar Bandi <sarveshwar.bandi@emulex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>