xen/balloon: Subtract from xen_released_pages the count that is populated.

We did not take into account that xen_released_pages would be
used outside the initial E820 parsing code. As such we would
did not subtract from xen_released_pages the count of pages
that we had populated back (instead we just did a simple
extra_pages = released - populated).

The balloon driver uses xen_released_pages to set the initial
current_pages count.  If this is wrong (too low) then when a new
(higher) target is set, the balloon driver will request too many pages
from Xen."

This fixes errors such as:

(XEN) memory.c:133:d0 Could not allocate order=0 extent: id=0 memflags=0 (51 of 512)
during bootup and
free_memory            : 0

where the free_memory should be 128.
Acked-by: David Vrabel <david.vrabel@citrix.com>
[v1: Per David's review made the git commit better]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/setup.c

@@ -371,7 +371,8 @@ char * __init xen_memory_setup(void)
 	populated = xen_populate_chunk(map, memmap.nr_entries,
 			max_pfn, &last_pfn, xen_released_pages);
 
-	extra_pages += (xen_released_pages - populated);
+	xen_released_pages -= populated;
+	extra_pages += xen_released_pages;
 
 	if (last_pfn > max_pfn) {
 		max_pfn = min(MAX_DOMAIN_PAGES, last_pfn);