[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/net/ipv6/udp.c b/net/ipv6/udp.c
index 0d22008..bb45b9b 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -536,7 +536,7 @@
 	uh->check = csum_ipv6_magic(&fl->fl6_src, &fl->fl6_dst,
 				    up->len, fl->proto, csum   );
 	if (uh->check == 0)
-		uh->check = -1;
+		uh->check = CSUM_MANGLED_0;
 
 	err = ip6_push_pending_frames(sk);
 out: