io_uring/io-wq: do not use bogus hash value

Previously, the `hash` variable was initialized with `-1` and only
updated by io_get_next_work() if the current work was hashed. Commit
60cf46ae6054 ("io-wq: hash dependent work") changed this to always
call io_get_work_hash() even if the work was not hashed. This caused
the `hash != -1U` check to always be true, adding some overhead for
the `hash->wait` code.

This patch fixes the regression by checking the `IO_WQ_WORK_HASHED`
flag.

Perf diff for a flood of `IORING_OP_NOP` with `IOSQE_ASYNC`:

38.55% -1.57% [kernel.kallsyms] [k] queued_spin_lock_slowpath
6.86% -0.72% [kernel.kallsyms] [k] io_worker_handle_work
0.10% +0.67% [kernel.kallsyms] [k] put_prev_entity
1.96% +0.59% [kernel.kallsyms] [k] io_nop_prep
3.31% -0.51% [kernel.kallsyms] [k] try_to_wake_up
7.18% -0.47% [kernel.kallsyms] [k] io_wq_free_work

Fixes: 60cf46ae6054 ("io-wq: hash dependent work")
Cc: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Link: https://lore.kernel.org/r/20250128133927.3989681-6-max.kellermann@ionos.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>