io_uring: count allocated requests

Keep track of the number requests a ring currently has allocated (and
not freed), it'll be needed in the next patch.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/c8f8308294dc2a1cb8925d984d937d4fc14ab5d4.1746788718.git.asml.silence@gmail.com


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