ext2: Explicitly assign values to on-disk enum of filetypes

It is somewhat dangerous to use a straight enum here, because this will
reassign values of later variables if one of the earlier ones is removed.

Signed-off-by: Jan Blunck <jblunck@suse.de>
Cc: Andreas Dilger <adilger@sun.com>
Signed-off-by: Jan Kara <jack@suse.cz>