fs: ext2: update how memory is allocated in ext2

- `ext2_file` and `ext2_dir` merged into one struct and allocated from
  dedicated slab
- `ext2_inode` allocated from dedicated slab
- `ext2_disk_dentry` allocated from heap (because their size is not
  constant)
- `ext2_bgroup` and `ext2_disk_superblock` statically allocated

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