[NET]: Make mangling a checksum (0 -> 0xffff on the wire) explicit.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/checksum.h b/include/net/checksum.h
index 2b3c8dc..1242461 100644
--- a/include/net/checksum.h
+++ b/include/net/checksum.h
@@ -92,4 +92,5 @@
 	return (__force __wsum)n;
 }
 
+#define CSUM_MANGLED_0 ((__force __sum16)0xffff)
 #endif