bridge: provide a mtu() method for fake_dst_ops

Commit 618f9bc7

 (net: Move mtu handling down to the protocol
depended handlers) forgot the bridge netfilter case, adding a NULL
dereference in ip_fragment().

Reported-by: Chris Boot <bootc@bootc.net>
CC: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>