selftests: iou-zcrx: Get the page size at runtime

Use the API `sysconf()` to query page size at runtime, instead of using
hard code number 4096.

And use `posix_memalign` to allocate the page size aligned momory.

Signed-off-by: Haiyue Wang <haiyuewa@163.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: David Wei <dw@davidwei.uk>
Link: https://patch.msgid.link/20250419141044.10304-1-haiyuewa@163.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>