io_uring: Use system_dfl_wq instead of system_unbound_wq

io_uring: Use system_dfl_wq instead of system_unbound_wq

Commit de7341ffe49e ("io_uring: switch normal task_work to a mpscq")
added a use of system_unbound_wq, which is deprecated in favor of
system_dfl_wq added by commit 128ea9f6ccfb ("workqueue: Add
system_percpu_wq and system_dfl_wq"). An upcoming warning in the
workqueue tree flags this with:

  workqueue: work func io_tctx_fallback_work enqueued on deprecated workqueue. Use system_{percpu|dfl}_wq instead.

Switch to system_dfl_wq to clear up the warning.

Fixes: de7341ffe49e ("io_uring: switch normal task_work to a mpscq")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260616-io_uring-fix-wq-warning-v1-1-cfc9d934eedb@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>