udp6: respect IPV6_DONTFRAG sockopt in case there are pending frames

if up->pending != 0 dontfrag is left with default value -1. That
causes that application that do:
sendto len>mtu flag MSG_MORE
sendto len>mtu flag 0
will receive EMSGSIZE errno as the result of the second sendto.

This patch fixes it by respecting IPV6_DONTFRAG socket option.

introduced by:
commit 4b340ae2 "IPv6: Complete IPV6_DONTFRAG support"
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/udp.c

@@ -1225,9 +1225,6 @@ int udpv6_sendmsg(struct kiocb *iocb, struct sock *sk,
 	if (tclass < 0)
 		tclass = np->tclass;
 
-	if (dontfrag < 0)
-		dontfrag = np->dontfrag;
-
 	if (msg->msg_flags&MSG_CONFIRM)
 		goto do_confirm;
 back_from_confirm:
@@ -1246,6 +1243,8 @@ int udpv6_sendmsg(struct kiocb *iocb, struct sock *sk,
 	up->pending = AF_INET6;
 
 do_append_data:
+	if (dontfrag < 0)
+		dontfrag = np->dontfrag;
 	up->len += ulen;
 	getfrag  =  is_udplite ?  udplite_getfrag : ip_generic_getfrag;
 	err = ip6_append_data(sk, getfrag, msg->msg_iov, ulen,