xen/balloon: remove BUG_ON in increase_reservation

The BUG_ON in increase_reservation is wrong as we have P2M entry
ballooned out page set to balloon scratch page, so it might have a valid
P2M entry at that point.
Signed-off-by: NWei Liu <wei.liu2@citrix.com>
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>

drivers/xen/balloon.c

@@ -349,8 +349,6 @@ static enum bp_state increase_reservation(unsigned long nr_pages)
 		BUG_ON(page == NULL);
 
 		pfn = page_to_pfn(page);
-		BUG_ON(!xen_feature(XENFEAT_auto_translated_physmap) &&
-		       phys_to_machine_mapping_valid(pfn));
 
 		set_phys_to_machine(pfn, frame_list[i]);