ext4: Improve ext4 Kconfig test

Now that ext4 driver must be used to access ext3 filesystems, improve
the Kconfig help text to better explain that using ext4 driver to access
the filesystem is fully compatible with the old ext3 driver.

Acked-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jan Kara <jack@suse.com>