sctp: add support for RPS and RFS

This patch adds what's missing to properly support RPS and RFS on SCTP,
as some of it is already implemented in common calls.

Having support for RPS and RFS allows better scaling specially because
not all NICs support hashing SCTP headers.

Save the hash right when we dequeue a skb from inqueue so we do it only
once per skb instead of per chunk. New sockets will then inherit the
hash through sctp_copy_sock().

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>