mm, swap: remove fragment clusters counter

mm, swap: remove fragment clusters counter

It was used for calculating the iteration number when the swap allocator
wants to scan the whole fragment list.  Now the allocator only scans one
fragment cluster at a time, so no one uses this counter anymore.

Remove it as a cleanup; the performance change is marginal:

Build linux kernel using 10G ZRAM, make -j96, defconfig with 2G cgroup
memory limit, on top of tmpfs, 64kB mTHP enabled:

Before:  sys time: 6278.45s
After:   sys time: 6176.34s

Change to 8G ZRAM:

Before:  sys time: 5572.85s
After:   sys time: 5531.49s

Link: https://lkml.kernel.org/r/20250806161748.76651-3-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Chris Li <chrisl@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>