ext4: convert sbi->s_mb_free_pending to atomic_t

Previously, s_md_lock was used to protect s_mb_free_pending during
modifications, while smp_mb() ensured fresh reads, so s_md_lock just
guarantees the atomicity of s_mb_free_pending. Thus we optimized it by
converting s_mb_free_pending into an atomic variable, thereby eliminating
s_md_lock and minimizing lock contention. This also prepares for future
lockless merging of free extents.

Following this modification, s_md_lock is exclusively responsible for
managing insertions and deletions within s_freed_data_list, along with
operations involving list_splice.

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 | 19628 | 20043 (+2.1%)  | 320885 | 314331 (-2.0%) |
|mb_optimize_scan=1 | 7129  | 7290  (+2.2%)  | 321275 | 324226 (+0.9%) |

|CPU: AMD 9654 * 2  |          P96           |             P1          |
|Memory: 1536GB     |------------------------|-------------------------|
|960GB SSD (1GB/s)  | base  |    patched     | base   |    patched     |
|-------------------|-------|----------------|--------|----------------|
|mb_optimize_scan=0 | 53760 | 54999 (+2.3%)  | 213145 | 214380 (+0.5%) |
|mb_optimize_scan=1 | 12716 | 13497 (+6.1%)  | 215262 | 216276 (+0.4%) |

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/20250714130327.1830534-9-libaokun1@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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