[BRIDGE]: Unshare skb upon entry

Due to the special location of the bridging hook, it should never see a
shared packet anyway (certainly not with any in-kernel code).  So it
makes sense to unshare the skb there if necessary as that will greatly
simplify the code below it (in particular, netfilter).
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/bridge/br_input.c

@@ -126,6 +126,10 @@ struct sk_buff *br_handle_frame(struct net_bridge_port *p, struct sk_buff *skb)
 	if (!is_valid_ether_addr(eth_hdr(skb)->h_source))
 		goto drop;
 
+	skb = skb_share_check(skb, GFP_ATOMIC);
+	if (!skb)
+		return NULL;
+
 	if (unlikely(is_link_local(dest))) {
 		/* Pause frames shouldn't be passed up by driver anyway */
 		if (skb->protocol == htons(ETH_P_PAUSE))