ext4: merge freed extent with existing extents before insertion

ext4: merge freed extent with existing extents before insertion

Attempt to merge ext4_free_data with already inserted free extents prior
to adding new ones. This strategy drastically cuts down the number of
times locks are held.

For example, if prev, new, and next extents are all mergeable, the existing
code (before this patch) requires acquiring the s_md_lock three times:

  prev merge into new and free prev // hold lock
  next merge into new and free next // hold lock
  insert new // hold lock

After the patch, it only needs to be acquired once:

  new merge into next and free new // no lock
  next merge into prev and free next // hold lock

Performance test data follows:

Test: Running will-it-scale/fallocate2 on CPU-bound containers.
Observation: Average fallocate operations per container per second.

|CPU: Kunpeng 920   |          P80           |            P1           |
|Memory: 512GB      |------------------------|-------------------------|
|960GB SSD (0.5GB/s)| base  |    patched     | base   |    patched     |
|-------------------|-------|----------------|--------|----------------|
|mb_optimize_scan=0 | 20043 | 20097 (+0.2%)  | 314331 | 316141 (+0.5%) |
|mb_optimize_scan=1 | 7290  | 13318 (+87.4%) | 324226 | 325273 (+0.3%) |

|CPU: AMD 9654 * 2  |          P96           |             P1          |
|Memory: 1536GB     |------------------------|-------------------------|
|960GB SSD (1GB/s)  | base  |    patched     | base   |    patched     |
|-------------------|-------|----------------|--------|----------------|
|mb_optimize_scan=0 | 54999 | 53603 (-2.5%)  | 214380 | 214243 (-0.06%)|
|mb_optimize_scan=1 | 13497 | 20887 (+54.6%) | 216276 | 213632 (-1.2%) |

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20250714130327.1830534-10-libaokun1@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>