fstests: generic/746: update the parser to handle block group tree

fstests: generic/746: update the parser to handle block group tree

[FALSE ALERT]
The test case will fail on btrfs if the new block-group-tree feature is
enabled:

FSTYP         -- btrfs
PLATFORM      -- Linux/x86_64 btrfs-vm 6.18.0-rc6-custom+ #321 SMP PREEMPT_DYNAMIC Sun Nov 23 16:34:33 ACDT 2025
MKFS_OPTIONS  -- -O block-group-tree /dev/mapper/test-scratch1
MOUNT_OPTIONS -- /dev/mapper/test-scratch1 /mnt/scratch

generic/746 44s ... [failed, exit status 1]- output mismatch (see xfstests-dev/results//generic/746.out.bad)
    --- tests/generic/746.out	2024-06-27 13:55:51.286338519 +0930
    +++ xfstests-dev/results//generic/746.out.bad	2025-11-28 07:47:17.039827837 +1030
    @@ -2,4 +2,4 @@
     Generating garbage on loop...done.
     Running fstrim...done.
     Detecting interesting holes in image...done.
    -Comparing holes to the reported space from FS...done.
    +Comparing holes to the reported space from FS...Sectors 256-2111 are not marked as free!
    ...
    (Run 'diff -u xfstests-dev/tests/generic/746.out xfstests-dev/results//generic/746.out.bad'  to see the entire diff)

[CAUSE]
Sectors [256, 2048) are the from the reserved first 1M free space.
Sectors [2048, 2112) are the leading free space in the chunk tree.
Sectors [2112, 2144) is the first tree block in the chunk tree.

However the reported free sectors from get_free_sectors() looks like this:

  2144 10566
  10688 11711
  ...

Note that there should be a free sector range in [2048, 2112) but it's
not reported in get_free_sectors().

The get_free_sectors() call is fs dependent, and for btrfs it's using
parse-extent-tree.awk script to handle the extent tree dump.

The script uses BLOCK_GROUP_ITEM items to detect the beginning of a
block group so that it can calculate the hole between the beginning of a
block group and the first data/metadata item.

However block-group-tree feature moves BLOCK_GROUP_ITEM items to a
dedicated tree, making the existing script unable to parse the free
space at the beginning of a block group.

[FIX]
Introduce a new script, parse-free-space.py, that accepts two tree
dumps:

- block group tree dump
  If the fs has block-group-tree feature, it's the block group tree
  dump.
  Otherwise the regular extent tree dump is enough.

- extent tree dump
  The usual extent tree dump.

With a dedicated block group tree dump, the script can correctly handle
the beginning part of free space, no matter if block-group-tree feature
is enabled or not.

And with this parser, the old parse-extent-tree.awk can be retired.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>