[IPV6]: Fix OOPS when using IPV6_ADDRFORM

This causes sk->sk_prot to change, which makes the socket
release free the sock into the wrong SLAB cache.  Fix this
by introducing sk_prot_creator so that we always remember
where the sock came from.

Signed-off-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: David S. Miller <davem@davemloft.net>