io_uring/zcrx: separate niov number from pages

A preparation patch that separates the number of pages / folios from
the number of niovs. They will not match in the future to support huge
pages, improved dma mapping and/or larger chunk sizes.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/0780ac966ee84200385737f45bb0f2ada052392b.1743848231.git.asml.silence@gmail.com


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