io_uring/zcrx: split out memory holders from area

io_uring/zcrx: split out memory holders from area

In the data path users of struct io_zcrx_area don't need to know what
kind of memory it's backed by. Only keep there generic bits in there and
and split out memory type dependent fields into a new structure. It also
logically separates the step that actually imports the memory, e.g.
pinning user pages, from the generic area initialisation.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/b60fc09c76921bf69e77eb17e07eb4decedb3bf4.1746097431.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>