io_uring/io-wq: cache work->flags in variable

io_uring/io-wq: cache work->flags in variable

This eliminates several redundant atomic reads and therefore reduces
the duration the surrounding spinlocks are held.

In several io_uring benchmarks, this reduced the CPU time spent in
queued_spin_lock_slowpath() considerably:

io_uring benchmark with a flood of `IORING_OP_NOP` and `IOSQE_ASYNC`:

    38.86%     -1.49%  [kernel.kallsyms]  [k] queued_spin_lock_slowpath
     6.75%     +0.36%  [kernel.kallsyms]  [k] io_worker_handle_work
     2.60%     +0.19%  [kernel.kallsyms]  [k] io_nop
     3.92%     +0.18%  [kernel.kallsyms]  [k] io_req_task_complete
     6.34%     -0.18%  [kernel.kallsyms]  [k] io_wq_submit_work

HTTP server, static file:

    42.79%     -2.77%  [kernel.kallsyms]     [k] queued_spin_lock_slowpath
     2.08%     +0.23%  [kernel.kallsyms]     [k] io_wq_submit_work
     1.19%     +0.20%  [kernel.kallsyms]     [k] amd_iommu_iotlb_sync_map
     1.46%     +0.15%  [kernel.kallsyms]     [k] ep_poll_callback
     1.80%     +0.15%  [kernel.kallsyms]     [k] io_worker_handle_work

HTTP server, PHP:

    35.03%     -1.80%  [kernel.kallsyms]     [k] queued_spin_lock_slowpath
     0.84%     +0.21%  [kernel.kallsyms]     [k] amd_iommu_iotlb_sync_map
     1.39%     +0.12%  [kernel.kallsyms]     [k] _copy_to_iter
     0.21%     +0.10%  [kernel.kallsyms]     [k] update_sd_lb_stats

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Link: https://lore.kernel.org/r/20250128133927.3989681-5-max.kellermann@ionos.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>