btrfs: tree-checker: Add extent items check

Finally, we are going to add tree-checker support for extent items, which includes:
- EXTENT_ITEM/METADATA_ITEM
Which futher contains inline backrefs of:
* TREE_BLOCK_REF
* SHARED_BLOCK_REF
* EXETNT_DATA_REF
* SHARED_DATA_REF

- TREE_BLOCK_REF
- SHARED_BLOCK_REF
- EXTENT_DATA_REF
- SHARED_DATA_REF
Keyed version of the above types

The complexity of the on-disk format can be found in the first patch, which contains a basic introduction as comment.

Hidden pitfalls are everywhere, e.g. inlined EXTENT_DATA_REF don't use iref->offset, but put its own data at iref->offset.
But SHARED_DATA_REF uses iref->offset, and put extra data after iref.

Such on-disk layout makes sense, but definitely a mess to read. Thankfully we at least have print-tree code from btrfs-progs as a reference.

Qu Wenruo (3):
btrfs: tree-checker: Add EXTENT_ITEM and METADATA_ITEM check
btrfs: tree-checker: Add simple keyed refs check
btrfs: tree-checker: Add EXTENT_DATA_REF check

 fs/btrfs/ctree.h        |   1 +
 fs/btrfs/extent-tree.c  |   2 +-
 fs/btrfs/tree-checker.c | 335 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 337 insertions(+), 1 deletion(-)

2.22.0