[BR_NETFILTER]: Fix leak if skb traverses > 1 bridge

Call nf_bridge_put() before allocating a new nf_bridge structure and
potentially overwriting the pointer to a previously allocated one.
This fixes a memory leak which can occur when the bridge topology
allows for an skb to traverse more than one bridge.

Signed-off-by: David Kimdon <david.kimdon@devicescape.com>
Signed-off-by: David S. Miller <davem@davemloft.net>