xen/arm: pfn_to_mfn and mfn_to_pfn return the argument if nothing is in the p2m

Some common Xen drivers, like balloon.c, call pfn_to_mfn and mfn_to_pfn
even for autotranslate guests, expecting the argument back.
The following commit broke these drivers by changing the behavior of
pfn_to_mfn and mfn_to_pfn:

commit 4a19138c6505e224d9f4cc2fe9ada9188d7100ea
Author: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Date:   Thu Oct 17 16:22:27 2013 +0000

    arm/xen,arm64/xen: introduce p2m

They now return INVALID_P2M_ENTRY if Linux doesn't actually know what is
the mfn backing a pfn or what is the pfn corresponding to an mfn.
Fix the regression by switching to the old behavior.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Tested-by: Ian Campbell <ian.campbell@citrix.com>

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

@@ -46,10 +46,7 @@ static inline unsigned long pfn_to_mfn(unsigned long pfn)
 			return mfn;
 	}
 
-	if (xen_initial_domain())
-		return pfn;
-	else
-		return INVALID_P2M_ENTRY;
+	return pfn;
 }
 
 static inline unsigned long mfn_to_pfn(unsigned long mfn)
@@ -62,10 +59,7 @@ static inline unsigned long mfn_to_pfn(unsigned long mfn)
 			return pfn;
 	}
 
-	if (xen_initial_domain())
-		return mfn;
-	else
-		return INVALID_P2M_ENTRY;
+	return mfn;
 }
 
 #define mfn_to_local_pfn(mfn) mfn_to_pfn(mfn)