powerpc/64: Fix checksum folding in csum_add()

Paul's patch to fix checksum folding, commit b492f7e4 ("powerpc/64:
Fix checksum folding in csum_tcpudp_nofold and ip_fast_csum_nofold")
missed a case in csum_add(). Fix it.
Signed-off-by: NShile Zhang <shile.zhang@nokia.com>
Acked-by: NPaul Mackerras <paulus@ozlabs.org>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/checksum.h

@@ -112,7 +112,7 @@ static inline __wsum csum_add(__wsum csum, __wsum addend)
 
 #ifdef __powerpc64__
 	res += (__force u64)addend;
-	return (__force __wsum)((u32)res + (res >> 32));
+	return (__force __wsum) from64to32(res);
 #else
 	asm("addc %0,%0,%1;"
 	    "addze %0,%0;"