Do not attempt to report proc event if NOTE_EXIT has already been received.

This fixes a race condition (specifically with signal events) that could
lead to the kn being re-inserted into the list after it has been destroyed,
which is not something we want to happen.

PR:		kern/58258

sys/kern/kern_event.c

@@ -279,6 +279,13 @@ filt_proc(struct knote *kn, long hint)
 		return (1);
 	}
 
+	/*
+	 * Process will already be reported as gone.
+	 * Do not report anything else, as the knote will be destroyed soon.
+	 */
+	if (kn->kn_status & KN_DETACHED)
+		return (0);
+
 	/*
 	 * process forked, and user wants to track the new process,
 	 * so attach a new knote to it, and immediately report an