[PACKET]: Fix skb->cb clobbering between aux and sockaddr

Both aux data and sockaddr tries to use the same buffer which
obviously doesn't work.  We just happen to have 4 bytes free in
the skb->cb if you take away the maximum length of sockaddr_ll.
That's just enough to store the one piece of info from aux data
that we can't generate at recvmsg(2) time.

This is what the following patch does.

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