io_uring/mock: support for async read/write

Let the user to specify a delay to read/write request. io_uring will
start a timer, return -EIOCBQUEUED and complete the request
asynchronously after the delay pass.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/38f9d2e143fda8522c90a724b74630e68f9bbd16.1750599274.git.asml.silence@gmail.com


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