[PATCH] fix oops in fork() cleanup path

It will oops on an error path if the thread being forked is a process with
a NULL mm.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/fork.c

@@ -1146,7 +1146,8 @@ static task_t *copy_process(unsigned long clone_flags,
 bad_fork_cleanup_namespace:
 	exit_namespace(p);
 bad_fork_cleanup_mm:
-	mmput(p->mm);
+	if (p->mm)
+		mmput(p->mm);
 bad_fork_cleanup_signal:
 	exit_signal(p);
 bad_fork_cleanup_sighand: