io_uring: fix a memory leak of buffer group list on exit

If we use a buffer group ID that is large enough to require io_uring
to allocate it, then we don't correctly free it if the cleanup is
deferred to the ring exit. The explicit removal paths are fine.

Fixes: 9cfc7e94

 ("io_uring: get rid of hashed provided buffer groups")
Signed-off-by: Jens Axboe <axboe@kernel.dk>