Fix information leakage.

Without this fix it is possible to cheat policies like:
- sysctl security.bsd.see_other_[gu]ids=0,
- mac_seeotheruids(4),
- jail(2)
and get full processes list with their arguments.

This problem exists from revision 1.62 of kern_proc.c when it was
introduced.

Reviewed by:	nectar, rwatson.

sys/kern/kern_proc.c

@@ -1106,11 +1106,16 @@ sysctl_kern_proc_args(SYSCTL_HANDLER_ARGS)
 	if (!p)
 		return (ESRCH);
 
-	if ((!ps_argsopen) && (error = p_cansee(curthread, p))) {
+	if ((error = p_cansee(curthread, p)) != 0) {
 		PROC_UNLOCK(p);
 		return (error);
 	}
 
+	if (!ps_argsopen) {
+		PROC_UNLOCK(p);
+		return (EPERM);
+	}
+
 	if (req->newptr && curproc != p) {
 		PROC_UNLOCK(p);
 		return (EPERM);