Revert "kernfs: remove unnecessary NULL check in __kernfs_remove()"

This reverts commit 88533f99.

Tejun writes:
        I'm sorry but can you please revert the whole series?
        get_active() waiting while a node is deactivated has potential
        to lead to deadlock and that deactivate/reactivate interface is
        something fundamentally flawed and that cgroup will have to work
        with the remove_self() like everybody else.  IOW, I think the
        first posting was correct.

Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/kernfs/dir.c

@@ -920,6 +920,9 @@ static void __kernfs_remove(struct kernfs_node *kn)
 
 	lockdep_assert_held(&kernfs_mutex);
 
+	if (!kn)
+		return;
+
 	pr_debug("kernfs %s: removing\n", kn->name);
 
 	__kernfs_deactivate(kn);