io_uring/io-wq: add io_worker.acct pointer

io_uring/io-wq: add io_worker.acct pointer

This replaces the `IO_WORKER_F_BOUND` flag.  All code that checks this
flag is not interested in knowing whether this is a "bound" worker;
all it does with this flag is determine the `io_wq_acct` pointer.  At
the cost of an extra pointer field, we can eliminate some fragile
pointer arithmetic.  In turn, the `create_index` and `index` fields
are not needed anymore.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Link: https://lore.kernel.org/r/20250128133927.3989681-3-max.kellermann@ionos.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>