powerpc: add support for csum_add()

The C version of csum_add() as defined in include/net/checksum.h gives
the following assembly in ppc32:
       0:       7c 04 1a 14     add     r0,r4,r3
       4:       7c 64 00 10     subfc   r3,r4,r0
       8:       7c 63 19 10     subfe   r3,r3,r3
       c:       7c 63 00 50     subf    r3,r3,r0
and the following in ppc64:
   0xc000000000001af8 <+0>:	add     r3,r3,r4
   0xc000000000001afc <+4>:	cmplw   cr7,r3,r4
   0xc000000000001b00 <+8>:	mfcr    r4
   0xc000000000001b04 <+12>:	rlwinm  r4,r4,29,31,31
   0xc000000000001b08 <+16>:	add     r3,r4,r3
   0xc000000000001b0c <+20>:	clrldi  r3,r3,32
   0xc000000000001b10 <+24>:	blr

include/net/checksum.h also offers the possibility to define an arch
specific function.  This patch provides a specific csum_add() inline
function.
Signed-off-by: NChristophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: NScott Wood <scottwood@freescale.com>

arch/powerpc/include/asm/checksum.h

@@ -130,6 +130,22 @@ static inline __sum16 csum_tcpudp_magic(__be32 saddr, __be32 daddr,
 	return csum_fold(csum_tcpudp_nofold(saddr, daddr, len, proto, sum));
 }
 
+#define HAVE_ARCH_CSUM_ADD
+static inline __wsum csum_add(__wsum csum, __wsum addend)
+{
+#ifdef __powerpc64__
+	u64 res = (__force u64)csum;
+
+	res += (__force u64)addend;
+	return (__force __wsum)((u32)res + (res >> 32));
+#else
+	asm("addc %0,%0,%1;"
+	    "addze %0,%0;"
+	    : "+r" (csum) : "r" (addend));
+	return csum;
+#endif
+}
+
 #endif
 #endif /* __KERNEL__ */
 #endif