fs: ext2: Use disk structs to access data on disk

This commits changes how data on the disk is accessed. There are disk
structures which are packed and their fields are stored in little endian
byte order. To use data in the program structures it has to be translated
from little endian to cpu endianness.

Signed-off-by: Franciszek Zdobylak <fzdobylak@antmicro.com>