ext4: utilize multiple global goals to reduce contention

When allocating data blocks, if the first try (goal allocation) fails and
stream allocation is on, it tries a global goal starting from the last
group we used (s_mb_last_group). This helps cluster large files together
to reduce free space fragmentation, and the data block contiguity also
accelerates write-back to disk.

However, when multiple processes allocate blocks, having just one global
goal means they all fight over the same group. This drastically lowers
the chances of extents merging and leads to much worse file fragmentation.

To mitigate this multi-process contention, we now employ multiple global
goals, with the number of goals being the minimum between the number of
possible CPUs and one-quarter of the filesystem's total block group count.

To ensure a consistent goal for each inode, we select the corresponding
goal by taking the inode number modulo the total number of goals.

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 | 9636  | 19628 (+103%)  | 337597 | 320885 (-4.9%) |
|mb_optimize_scan=1 | 4834  | 7129  (+47.4%) | 341440 | 321275 (-5.9%) |

|CPU: AMD 9654 * 2  |          P96           |             P1          |
|Memory: 1536GB     |------------------------|-------------------------|
|960GB SSD (1GB/s)  | base  |    patched     | base   |    patched     |
|-------------------|-------|----------------|--------|----------------|
|mb_optimize_scan=0 | 22341 | 53760 (+140%)  | 219707 | 213145 (-2.9%) |
|mb_optimize_scan=1 | 9177  | 12716 (+38.5%) | 215732 | 215262 (+0.2%) |

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20250714130327.1830534-6-libaokun1@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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