sctp: add SCTP_PR_SUPPORTED on sctp sockopt

According to section 4.5 of rfc7496, prsctp_enable should be per asoc.
We will add prsctp_enable to both asoc and ep, and replace the places
where it used net.sctp->prsctp_enable with asoc->prsctp_enable.

ep->prsctp_enable will be initialized with net.sctp->prsctp_enable, and
asoc->prsctp_enable will be initialized with ep->prsctp_enable. We can
also modify it's value through sockopt SCTP_PR_SUPPORTED.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/uapi/linux/sctp.h b/include/uapi/linux/sctp.h
index ce70fe6..aa08906 100644
--- a/include/uapi/linux/sctp.h
+++ b/include/uapi/linux/sctp.h
@@ -112,6 +112,7 @@
 #define SCTP_SOCKOPT_CONNECTX	110		/* CONNECTX requests. */
 #define SCTP_SOCKOPT_CONNECTX3	111	/* CONNECTX requests (updated) */
 #define SCTP_GET_ASSOC_STATS	112	/* Read only */
+#define SCTP_PR_SUPPORTED	113
 
 /* These are bit fields for msghdr->msg_flags.  See section 5.1.  */
 /* On user space Linux, these live in <bits/socket.h> as an enum.  */