Fix fork failure case.

It would clear the segment registers in the parent, which
wasn't really intentional. Noticed by Andries Brouwer.

kernel/fork.c

@@ -1144,9 +1144,7 @@ static task_t *copy_process(unsigned long clone_flags,
 bad_fork_cleanup_namespace:
 	exit_namespace(p);
 bad_fork_cleanup_mm:
-	exit_mm(p);
-	if (p->active_mm)
-		mmdrop(p->active_mm);
+	mmput(mm);
 bad_fork_cleanup_signal:
 	exit_signal(p);
 bad_fork_cleanup_sighand: