io_uring: add helper to free all request caches

io_uring: add helper to free all request caches

We have three different ones, put it in a helper for easy calling. This
is in preparation for doing it outside of ring freeing as well. With
that in mind, also ensure that we do the proper locking for safe calling
from a context where the ring it still live.

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