btrfs: reclaim compression cache in shrinker-sized batches

[eeea2222]

Summary

Make the Btrfs compression page-pool shrinker reclaim only the requested shrinker batch under memory pressure instead of draining the whole cached page list.

Teardown behavior is preserved: calls without a shrink control still drain the full cache.

Why

The previous scan path drained the entire compression page cache once reclaim asked for objects. That can discard more cached pages than requested and cause avoidable reallocation churn in compressed Btrfs workloads.

Compatibility

No userspace ABI, module contract, exported symbol, Kconfig, or dependency changes. This is an internal shrinker behavior refinement.

Validation

• Passed targeted ARCH=um object build for fs/btrfs/compression.o.
• x86 object validation was attempted locally but blocked before compiling this object because the host environment lacked gelf.h, required by objtool.