net: Allow csum_add to be provided in arch

csum_add is really nothing more then add-with-carry which
can be implemented efficiently in some architectures.
Allow architecture to define this protected by HAVE_ARCH_CSUM_ADD.
Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/checksum.h

@@ -57,12 +57,14 @@ static __inline__ __wsum csum_and_copy_to_user
 }
 #endif
 
+#ifndef HAVE_ARCH_CSUM_ADD
 static inline __wsum csum_add(__wsum csum, __wsum addend)
 {
 	u32 res = (__force u32)csum;
 	res += (__force u32)addend;
 	return (__force __wsum)(res + (res < (__force u32)addend));
 }
+#endif
 
 static inline __wsum csum_sub(__wsum csum, __wsum addend)
 {