debugfs: debugfs_lookup() should return NULL if not found

Lots of callers of debugfs_lookup() were just checking NULL to see if
the file/directory was found or not.  By changing this in ff9fb72b
("debugfs: return error values, not NULL") we caused some subsystems to
easily crash.

Fixes: ff9fb72b ("debugfs: return error values, not NULL")
Reported-by: syzbot+b382ba6a802a3d242790@syzkaller.appspotmail.com
Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Jens Axboe <axboe@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/debugfs/inode.c

@@ -254,8 +254,8 @@ MODULE_ALIAS_FS("debugfs");
  * @parent: a pointer to the parent dentry of the file.
  *
  * This function will return a pointer to a dentry if it succeeds.  If the file
- * doesn't exist or an error occurs, %ERR_PTR(-ERROR) will be returned.  The
- * returned dentry must be passed to dput() when it is no longer needed.
+ * doesn't exist or an error occurs, %NULL will be returned.  The returned
+ * dentry must be passed to dput() when it is no longer needed.
  *
  * If debugfs is not enabled in the kernel, the value -%ENODEV will be
  * returned.
@@ -265,17 +265,17 @@ struct dentry *debugfs_lookup(const char *name, struct dentry *parent)
 	struct dentry *dentry;
 
 	if (IS_ERR(parent))
-		return parent;
+		return NULL;
 
 	if (!parent)
 		parent = debugfs_mount->mnt_root;
 
 	dentry = lookup_one_len_unlocked(name, parent, strlen(name));
 	if (IS_ERR(dentry))
-		return dentry;
+		return NULL;
 	if (!d_really_is_positive(dentry)) {
 		dput(dentry);
-		return ERR_PTR(-EINVAL);
+		return NULL;
 	}
 	return dentry;
 }