xen/p2m: Allocate p2m tracking pages on override

It is possible to add a p2m override on pages that are currently mapped
to INVALID_P2M_ENTRY; in particular, this will happen when using
ballooned pages in gntdev. This means that set_phys_to_machine must be
used instead of __set_phys_to_machine.
Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/p2m.c

@@ -671,7 +671,9 @@ int m2p_add_override(unsigned long mfn, struct page *page)
 	page->private = mfn;
 	page->index = pfn_to_mfn(pfn);
 
-	__set_phys_to_machine(pfn, FOREIGN_FRAME(mfn));
+	if (unlikely(!set_phys_to_machine(pfn, FOREIGN_FRAME(mfn))))
+		return -ENOMEM;
+
 	if (!PageHighMem(page))
 		/* Just zap old mapping for now */
 		pte_clear(&init_mm, address, ptep);
@@ -709,7 +711,7 @@ int m2p_remove_override(struct page *page)
 	spin_lock_irqsave(&m2p_override_lock, flags);
 	list_del(&page->lru);
 	spin_unlock_irqrestore(&m2p_override_lock, flags);
-	__set_phys_to_machine(pfn, page->index);
+	set_phys_to_machine(pfn, page->index);
 
 	if (!PageHighMem(page))
 		set_pte_at(&init_mm, address, ptep,