kernfs: fix potential NULL dereference in __kernfs_remove

When lockdep is enabled, lockdep_assert_held_write would
cause potential NULL pointer dereference.

Fix the following smatch warnings:

fs/kernfs/dir.c:1353 __kernfs_remove() warn: variable dereferenced before check 'kn' (see line 1346)

Fixes: 393c3714 ("kernfs: switch global kernfs_rwsem lock to per-fs lock")
Signed-off-by: Yushan Zhou <katrinzhou@tencent.com>
Link: https://lore.kernel.org/r/20220630082512.3482581-1-zys.zljxml@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/kernfs/dir.c

@@ -1343,14 +1343,17 @@ static void __kernfs_remove(struct kernfs_node *kn)
 {
 	struct kernfs_node *pos;
 
+	/* Short-circuit if non-root @kn has already finished removal. */
+	if (!kn)
+		return;
+
 	lockdep_assert_held_write(&kernfs_root(kn)->kernfs_rwsem);
 
 	/*
-	 * Short-circuit if non-root @kn has already finished removal.
 	 * This is for kernfs_remove_self() which plays with active ref
 	 * after removal.
 	 */
-	if (!kn || (kn->parent && RB_EMPTY_NODE(&kn->rb)))
+	if (kn->parent && RB_EMPTY_NODE(&kn->rb))
 		return;
 
 	pr_debug("kernfs %s: removing\n", kn->name);