xen/m2p: No need to catch exceptions when we know that there is no RAM

.. beyound what we think is the end of memory. However there might
be more System RAM - but assigned to a guest. Hence jump to the
M2P override check and consult.

[v1: Added Review-by tag]
Reviewed-by: NIan Campbell <ian.campbell@citrix.com>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/include/asm/xen/page.h

@@ -85,6 +85,10 @@ static inline unsigned long mfn_to_pfn(unsigned long mfn)
 	if (xen_feature(XENFEAT_auto_translated_physmap))
 		return mfn;
 
+	if (unlikely((mfn >> machine_to_phys_order) != 0)) {
+		pfn = ~0;
+		goto try_override;
+	}
 	pfn = 0;
 	/*
 	 * The array access can fail (e.g., device space beyond end of RAM).
@@ -92,7 +96,7 @@ static inline unsigned long mfn_to_pfn(unsigned long mfn)
 	 * but we must handle the fault without crashing!
 	 */
 	__get_user(pfn, &machine_to_phys_mapping[mfn]);
-
+try_override:
 	/*
 	 * If this appears to be a foreign mfn (because the pfn
 	 * doesn't map back to the mfn), then check the local override