io_uring: Add generic helper to allocate async data

This helper replaces io_alloc_async_data by using the folded allocation.
Do it in a header to allow the compiler to decide whether to inline.

Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
Link: https://lore.kernel.org/r/20241216204615.759089-3-krisman@suse.de


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