sctp: not allow to set asoc prsctp_enable by sockopt

As rfc7496#section4.5 says about SCTP_PR_SUPPORTED:

   This socket option allows the enabling or disabling of the
   negotiation of PR-SCTP support for future associations.  For existing
   associations, it allows one to query whether or not PR-SCTP support
   was negotiated on a particular association.

It means only sctp sock's prsctp_enable can be set.

Note that for the limitation of SCTP_{CURRENT|ALL}_ASSOC, we will
add it when introducing SCTP_{FUTURE|CURRENT|ALL}_ASSOC for linux
sctp in another patchset.

v1->v2:
  - drop the params.assoc_id check as Neil suggested.

Fixes: 28aa4c26 ("sctp: add SCTP_PR_SUPPORTED on sctp sockopt")
Reported-by: NYing Xu <yinxu@redhat.com>
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sctp/socket.c

@@ -3940,32 +3940,16 @@ static int sctp_setsockopt_pr_supported(struct sock *sk,
 					unsigned int optlen)
 {
 	struct sctp_assoc_value params;
-	struct sctp_association *asoc;
-	int retval = -EINVAL;
 
 	if (optlen != sizeof(params))
-		goto out;
-
-	if (copy_from_user(&params, optval, optlen)) {
-		retval = -EFAULT;
-		goto out;
-	}
-
-	asoc = sctp_id2assoc(sk, params.assoc_id);
-	if (asoc) {
-		asoc->prsctp_enable = !!params.assoc_value;
-	} else if (!params.assoc_id) {
-		struct sctp_sock *sp = sctp_sk(sk);
+		return -EINVAL;
 
-		sp->ep->prsctp_enable = !!params.assoc_value;
-	} else {
-		goto out;
-	}
+	if (copy_from_user(&params, optval, optlen))
+		return -EFAULT;
 
-	retval = 0;
+	sctp_sk(sk)->ep->prsctp_enable = !!params.assoc_value;
 
-out:
-	return retval;
+	return 0;
 }
 
 static int sctp_setsockopt_default_prinfo(struct sock *sk,