xen/balloon: Use the correct sizeof when declaring frame_list

The type of the item in frame_list is xen_pfn_t which is not an unsigned
long on ARM but an uint64_t.

With the current computation, the size of frame_list will be 2 *
PAGE_SIZE rather than PAGE_SIZE.

I bet it's just mistake when the type has been switched from "unsigned
long" to "xen_pfn_t" in commit 965c0aaa
"xen: balloon: use correct type for frame_list".

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>