io_uring/kbuf: move pinning of provided buffer ring into helper

In preparation for allowing the kernel to allocate the provided buffer
rings and have the application mmap it instead, abstract out the
current method of pinning and mapping the user allocated ring.

No functional changes intended in this patch.

Acked-by: Helge Deller <deller@gmx.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>