io_uring: add ring freeing helper

Commit 9c189eee upstream.

We do rings and sqes separately, move them into a helper that does both
the freeing and clearing of the memory.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>