xen/grant: Introduce helpers to split a page into grant

xen/grant: Introduce helpers to split a page into grant

Currently, a grant is always based on the Xen page granularity (i.e
4KB). When Linux is using a different page granularity, a single page
will be split between multiple grants.

The new helpers will be in charge of splitting the Linux page into grants
and call a function given by the caller on each grant.

Also provide an helper to count the number of grants within a given
contiguous region.

Note that the x86/include/asm/xen/page.h is now including
xen/interface/grant_table.h rather than xen/grant_table.h. It's
necessary because xen/grant_table.h depends on asm/xen/page.h and will
break the compilation. Furthermore, only definition in
interface/grant_table.h is required.

Signed-off-by: Julien Grall <[email protected]>
Reviewed-by: David Vrabel <[email protected]>
Reviewed-by: Stefano Stabellini <[email protected]>
Signed-off-by: David Vrabel <[email protected]>