net: ip_fragment: remove BRIDGE_NETFILTER mtu special handling

since commit d6b915e29f4adea9
("ip_fragment: don't forward defragmented DF packet") the largest
fragment size is available in the IPCB.

Therefore we no longer need to care about 'encapsulation'
overhead of stripped PPPOE/VLAN headers since ip_do_fragment
doesn't use device mtu in such cases.

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

diff --cc include/linux/netfilter_bridge.h
Simple merge

diff --cc net/bridge/br_netfilter.c
Simple merge

diff --cc net/ipv4/ip_output.c
Simple merge