io_uring/uring_cmd: Allocate async data through generic helper

This abstracts away the cache details and simplify the code.

Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
Link: https://lore.kernel.org/r/20241216204615.759089-6-krisman@suse.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>