io-wq: Move wq accounting to io_wq

Since we now have a single io_wqe per io_wq instead of per-node, and in
preparation to its removal, move the accounting into the parent
structure.

Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
Link: https://lore.kernel.org/r/20230322011628.23359-2-krisman@suse.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>