bridge: fix compile warning in net/bridge/br_netfilter.c

This patch fixes the following warning due to incompatible pointer
assignment:

net/bridge/br_netfilter.c: In function 'br_netfilter_rtable_init':
net/bridge/br_netfilter.c:116: warning: assignment from incompatible
pointer type

This warning is due to commit 4adf0af6
from July 30 (send correct MTU value in PMTU (revised)).

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>