fs:ext4:write: Store block device descriptor in file system structure

The device block descriptor (block_dev_desc_t) )shall be stored at
ext4 early code (at ext4fs_set_blk_dev in this case) to be available
for latter use (like put_ext4()).

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>