io_uring: replace list with stack for req caches

Replace struct list_head free_list serving for caching requests with
singly linked stack, which is faster.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/1bc942b82422fb2624b8353bd93aca183a022846.1632516769.git.asml.silence@gmail.com


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