btrfs: report congestion for NO_THRESHOLD workqueues - #11

Summary

Keep pending-work accounting for Btrfs workqueues even when they use NO_THRESHOLD, and let btrfs_workqueue_normal_congested() report congestion for those queues based on pending work versus online CPU count.

Dynamic max-active behavior for thresholded workqueues is unchanged.

Why

NO_THRESHOLD queues previously always reported uncongested, even when pending work was high. This hid backpressure from callers that use congestion as a signal.

Compatibility

No userspace ABI, module contract, exported symbol, Kconfig, or dependency changes. Existing workqueue interfaces remain unchanged.

Validation

• Passed targeted ARCH=um object build for fs/btrfs/async-thread.o.
• scripts/checkpatch.pl --no-tree --file fs/btrfs/async-thread.c passed.
• x86 object validation was attempted locally but blocked before compiling this object because the host environment lacked gelf.h, required by objtool.