io_uring: fix memory leak when removing provided buffers

In the Linux kernel, the following vulnerability has been resolved:

io_uring: fix memory leak when removing provided buffers

When removing provided buffers, io_buffer structs are not being disposed

of, leading to a memory leak. They can't be freed individually, because

they are allocated in page-sized groups. They need to be added to some

free list instead, such as io_buffers_cache. All callers already hold

the lock protecting it, apart from when destroying buffers, so had to

extend the lock there.