sctp: use inet_recvmsg to support sctp RFS well

Commit 486bdee0 ("sctp: add support for RPS and RFS")
saves skb->hash into sk->sk_rxhash so that the inet_* can
record it to flow table.

But sctp uses sock_common_recvmsg as .recvmsg instead
of inet_recvmsg, sock_common_recvmsg doesn't invoke
sock_rps_record_flow to record the flow. It may cause
that the receiver has no chances to record the flow if
it doesn't send msg or poll the socket.

So this patch fixes it by using inet_recvmsg as .recvmsg
in sctp.

Fixes: 486bdee0

 ("sctp: add support for RPS and RFS")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>