btrfs-progs: separate BLOCK_GROUP_TREE feature from extent-tree-v2

[CHANGELOG]
v3:
- Add the artificial dependency for block group tree
Now free-space-tree and no-holes must be enabled to use block group tree feature.
This is for both mkfs and btrfstune.

v2:
- Add the ability to convert to bg tree using btrfstune Unlike the original patches years ago, which goes one transaction to convert the full fs to bg tree, this new method goes multi-transaction.

After converting every 64 block groups, we will commit a transaction to avoid doing too much work in one trans.

And the new convert will have fs_info->last_convert_bg_byter to record which bgs have been converted.
This allows any bgs beyond above value to go new bg tree, while bgs before that threshold to go regular extent tree.

The only concern is, the new method is pretty large in one single patch (+427/-27), which is not easy to review.
I hope to get some feedback before adding the convert from bg tree (aka, convert from bg tree to regular extent tree).

The block group tree idea is introduced to greatly reduce mount time for large fs, over SEVERN years ago.

And 4 years ago, it's determined to let extent-tree-v2 to implement the feature.

However extent tree v2 still doesn't have a consistent on-disk format, nor any implementation on the real extent items, nor any tests on some independent sub-features.

I strongly doubt if that's a correct decision, especially considering there is really no dependency from extent tree v2 on this block group tree feature.

Not to mention this is against the common idea on progressive improvement.

So now is the time to revive the independent block group compat RO flag.

[CHANGE FROM EXTENT-TREE-V2]
- Don't store block group root into super block
There is no special reason for block group root to be stored in super block.

- Separate block-group-tree as a compat RO flag from extent-tree-v2 The change to extent tree is not affecting read-only opeartions. No reason to make it incompat.

- Fix a bug in extent-tree-v2 which doesn't initialize block group item correctly.
Since we're re-using the existing block group item structure, we should properly initialize chunk_objectid to 256, or tree block will reject it.

- Dynamically arrange the mkfs_block array
Instead a completely new array dedicated for extent-tree-v2, now we have proper helpers to add/delete block from the array on-the-fly.

[TODO]
- Add btrfstune support to convert from block-group-tree feature The infrastructure is already done.

Qu Wenruo (5):
btrfs-progs: mkfs: dynamically modify mkfs blocks array
btrfs-progs: don't save block group root into super block
btrfs-progs: separate block group tree from extent tree v2
btrfs-progs: btrfstune: add the ability to convert to block group tree
feature
btrfs-progs: mkfs: add artificial dependency for block group tree

 Documentation/btrfstune.rst |   5 +
 btrfstune.c                 | 148 ++++++++++++++++++++-
 check/main.c                |   8 +-
 cmds/inspect-dump-tree.c    |  11 --
 common/fsfeatures.c         |   8 ++
 common/fsfeatures.h         |   2 +
 kernel-shared/ctree.c       |   8 ++
 kernel-shared/ctree.h       |  55 ++++----
 kernel-shared/disk-io.c     | 103 +++++----------
 kernel-shared/disk-io.h     |   5 +-
 kernel-shared/extent-tree.c | 247 ++++++++++++++++++++++++++++++++++--
 kernel-shared/print-tree.c  |  11 +-
 mkfs/common.c               | 113 ++++++++++++++---
 mkfs/common.h               |  20 +--
 mkfs/main.c                 |  10 +-
 15 files changed, 578 insertions(+), 176 deletions(-)

2.37.0