bridge: netfilter: fix a memory leak

nf_bridge_alloc() always reset the skb->nf_bridge, so we should always
put the old one.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: Bart De Schuymer <bdschuym@pandora.be>
Signed-off-by: David S. Miller <davem@davemloft.net>