io_uring: Move from hlist to io_wq_work_node

io_uring: Move from hlist to io_wq_work_node

Having cache entries linked using the hlist format brings no benefit, and
also requires an unnecessary extra pointer address per cache entry.

Use the internal io_wq_work_node single-linked list for the internal
alloc caches (async_msghdr and async_poll)

This is required to be able to use KASAN on cache entries, since we do
not need to touch unused (and poisoned) cache entries when adding more
entries to the list.

Suggested-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/r/20230223164353.2839177-2-leitao@debian.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>