btrfs: tree-checker: Add ROOT_ITEM check

This patch will introduce ROOT_ITEM check, which includes:
- Key->objectid and key->offset check
  Currently only some easy check, e.g. 0 as rootid is invalid.

- Item size check
  Root item size is fixed.

- Generation checks
  Generation, generation_v2 and last_snapshot should not be greater than
  super generation + 1

- Level and alignment check
  Level should be in [0, 7], and bytenr must be aligned to sector size.

- Flags check

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=203261


Reported-by: Jungyeon Yoon <jungyeon.yoon@gmail.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>