[SCTP]: Fix couple of races between sctp_peeloff() and sctp_rcv().

Validate and update the sk in sctp_rcv() to avoid the race where an
assoc/ep could move to a different socket after we get the sk, but before
the skb is added to the backlog.

Also migrate the skb's in backlog queue to new sk when doing a peeloff.

Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>

include/net/sctp/sctp.h

@@ -175,6 +175,8 @@ void sctp_icmp_frag_needed(struct sock *, struct sctp_association *,
 void sctp_icmp_proto_unreachable(struct sock *sk,
 				 struct sctp_association *asoc,
 				 struct sctp_transport *t);
+void sctp_backlog_migrate(struct sctp_association *assoc,
+			  struct sock *oldsk, struct sock *newsk);
 
 /*
  *  Section:  Macros, externs, and inlines