[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: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>