xen/page_alloc: Keep away MFN 0 from the buddy allocator

xen/page_alloc: Keep away MFN 0 from the buddy allocator

Combining of buddies happens only such that the resulting larger buddy
is still order-aligned. To cross a zone boundary while merging, the
implication is that both the buddy [0, 2^n-1] and the buddy
[2^n, 2^(n+1)-1] are free.

Ideally we want to fix the allocator, but for now we can just prevent
adding the MFN 0 in the allocator to avoid merging across zone
boundaries.

On x86, the MFN 0 is already kept away from the buddy allocator. So the
bug can only happen on Arm platform where the first memory bank is
starting at 0.

As this is a specific to the allocator, the MFN 0 is removed in the common code
to cater all the architectures (current and future).

[Stefano: improve commit message]

Reported-by: Jeff Kubascik <jeff.kubascik@dornerworks.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Tested-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>