powerpc: mark xer clobbered in csum_add()

addc uses carry so xer is clobbered in csum_add()
Signed-off-by: NChristophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: NScott Wood <oss@buserror.net>

arch/powerpc/include/asm/checksum.h

@@ -141,7 +141,7 @@ static inline __wsum csum_add(__wsum csum, __wsum addend)
 #else
 	asm("addc %0,%0,%1;"
 	    "addze %0,%0;"
-	    : "+r" (csum) : "r" (addend));
+	    : "+r" (csum) : "r" (addend) : "xer");
 	return csum;
 #endif
 }