io_uring/zcrx: split common area map/unmap parts

Extract area type depedent parts of io_zcrx_[un]map_area from the
generic path. It'll be helpful once there are more area memory types
and not only user pages.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/50f6e893e2d20f937e628196cbf528d15f81c289.1746097431.git.asml.silence@gmail.com


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