[BR_NETFILTER]: Fix vlan-encapsulated fragmented IP traffic.

[BR_NETFILTER]: Fix vlan-encapsulated fragmented IP traffic.

When vlan-tagged fragmented IP traffic passes the bridging firewall and
ip_conntrack is loaded and iptables sees this IP traffic, an oops can
occur when trying to fragment the defragmented packets. This only
happens in the slow_path of ip_fragment().
The problem was reported, diagnosed and fixed by Adam Osuchowski and
Tomasz Dubinski.
When ip_fragment() is fragmenting an IP packet that's encapsulated, it has
to make sure there is enough head room for the encapsulating header.

The patch below fixes it. I saw no other way than to add some code to
ip_fragment(), but this extra code is located in the slow_path so it's
hardly ever executed.