io_uring/kbuf: use region api for pbuf rings

Convert internal parts of the provided buffer ring managment to the
region API. It's the last non-region mapped ring we have, so it also
kills a bunch of now unused memmap.c helpers.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/6c40cf7beaa648558acd4d84bc0fb3279a35d74b.1732886067.git.asml.silence@gmail.com


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