ext4: make the IS_EXT2_SB/IS_EXT3_SB checks more robust

Check for sb->s_type which is the right place to look at the file system
type, not the holder, which is just an implementation detail in the VFS
helpers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Message-Id: <20230802154131.2221419-6-hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>