[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/ipv4/udp.c b/net/ipv4/udp.c
index f9c4ed7..dc19ba1 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -441,7 +441,7 @@
 
 		uh->check = csum_tcpudp_magic(src, dst, len, IPPROTO_UDP, csum);
 		if (uh->check == 0)
-			uh->check = -1;
+			uh->check = CSUM_MANGLED_0;
 	}
 }
 
@@ -490,7 +490,7 @@
 	uh->check = csum_tcpudp_magic(fl->fl4_src, fl->fl4_dst, up->len,
 				      sk->sk_protocol, csum             );
 	if (uh->check == 0)
-		uh->check = -1;
+		uh->check = CSUM_MANGLED_0;
 
 send:
 	err = ip_push_pending_frames(sk);