ext4: add module aliases for ext2 and ext3

Add module aliases for ext2 and ext3 when CONFIG_EXT4_USE_FOR_EXT23 is
set.  This makes the existing user-space stuff like mkinitrd working
as is.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>