[gtpv2] Fix the overflow value for the sequence number (wmnsk#265)

Make the sequence number counter wrap around to
zero on reaching `0x800000` instead of
`0x1000000`.  The explanation why this value
should be used can be found in 3GPP TS 29.274,
section 7.6 ("Reliable Delivery of Signalling
Messages"), where it is mentioned that sequence
numbers with their most significant bit set to 1
should be used only for Command messages and for
requests which were triggered by Command messages,
as well as for Triggered Reply messages, and *not*
for requests which were not triggered by a Command
message (i.e. Initial Request messages).  It is
therefore assumed that if the sequence number
counter is initialized with zero, it is supposed
to be used when sending Initial Request messages
only.  That's why its value becoming `0x800000`
(and being incremented further) would in practice
change the type of the message and the way it's
supposed to be handled, which is incorrect
behavior.

gtpv2/conn.go

@@ -401,7 +401,7 @@ func (c *Conn) IncSequence() uint32 {
 	c.sequence++
 
 	// SequenceNumber is 3-octet long
-	if c.sequence > 0xffffff {
+	if c.sequence > 0x7fffff {
 		c.sequence = 0
 	}