xen/balloon: Don't rely on the page granularity is the same for Xen and Linux

xen/balloon: Don't rely on the page granularity is the same for Xen and Linux

For ARM64 guests, Linux is able to support either 64K or 4K page
granularity. Although, the hypercall interface is always based on 4K
page granularity.

With 64K page granularity, a single page will be spread over multiple
Xen frame.

To avoid splitting the page into 4K frame, take advantage of the
extent_order field to directly allocate/free chunk of the Linux page
size.

Note that PVMMU is only used for PV guest (which is x86) and the page
granularity is always 4KB. Some BUILD_BUG_ON has been added to ensure
that because the code has not been modified.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
---
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Note that two BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE) in code built
for the PV MMU code is kept in order to have at least one even if we
ever decide to drop of code section.

    Changes in v5:
        - Add Stefano's reviewed-by

    Changes in v4:
        - s/xen_page_to_pfn/page_to_xen_pfn/ based on the new naming
        - Use the field lru in the page to get a list of pages when
        decreasing the memory reservation. It avoids to use a static
        array to store the pages (see v3).
        - Update comment for EXTENT_ORDER.

    Changes in v3:
        - Fix errors reported by checkpatch.pl
        - s/mfn/gfn/ based on the new naming
        - Rather than splitting the page into 4KB chunk, use the
        extent_order field to allocate directly a Linux page size. This
        is avoid lots of code for no benefits.

    Changes in v2:
        - Use xen_apply_to_page to split a page in 4K chunk
        - It's not necessary to have a smaller frame list. Re-use
        PAGE_SIZE
        - Convert reserve_additional_memory to use XEN_... macro