[NETFILTER]: ip_nat_tftp: Fix expectation NAT

When a TFTP client is SNATed so that the port is also changed, the
port is never changed back for the expected connection.

Signed-off-by: Marcus Sundberg <[e-mail]>
Signed-off-by: Patrick McHardy <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/netfilter/ip_nat_tftp.c

···
 			 enum ip_conntrack_info ctinfo,
 			 struct ip_conntrack_expect *exp)
 {
-	exp->saved_proto.udp.port = exp->tuple.dst.u.tcp.port;
+	struct ip_conntrack *ct = exp->master;
+
+	exp->saved_proto.udp.port
+		= ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple.src.u.udp.port;
 	exp->dir = IP_CT_DIR_REPLY;
 	exp->expectfn = ip_nat_follow_master;
 	if (ip_conntrack_expect_related(exp) != 0)