[SCTP]: Fix compiler warning about const qualifiers

Fix 3 warnings about discarding const qualifiers:

net/sctp/ulpevent.c:862: warning: passing argument 1 of 'sctp_event2skb' discards qualifiers from pointer target type
net/sctp/sm_statefuns.c:4393: warning: passing argument 1 of 'SCTP_ASOC' discards qualifiers from pointer target type
net/sctp/socket.c:5874: warning: passing argument 1 of 'cmsg_nxthdr' discards qualifiers from pointer target type

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/sctp/ulpevent.h b/include/net/sctp/ulpevent.h
index 9bcfc12..7ea12e8 100644
--- a/include/net/sctp/ulpevent.h
+++ b/include/net/sctp/ulpevent.h
@@ -67,7 +67,7 @@
 };
 
 /* Retrieve the skb this event sits inside of. */
-static inline struct sk_buff *sctp_event2skb(struct sctp_ulpevent *ev)
+static inline struct sk_buff *sctp_event2skb(const struct sctp_ulpevent *ev)
 {
 	return container_of((void *)ev, struct sk_buff, cb);
 }