libgtp: Define retransmit QUEUE_SIZE relative to PDP_MAX (increase)

QUEUE_SIZE holds the number of pending transmitted messages
which can be handled concurrently.
Current value was 1024, same as PDP contexts (PDP_MAX). However, that
seems to be a quite low amount, which can be filled under certain
conditions, for instance if recovery procedure is triggered on the GSN
which is running full (around PDP_MAX pdp contexts created).
In this scenario, the GSN would need to send around PDP_MAX concurrent
messages (DeletePDPContextReq), which means the queue would very likely
end up being full.
Hence, let's define QUEUE_SIZE based on PDP_MAX, and set it to twice the
size to make sure it won't be filled in extreme conditions.

Change-Id: I6034b0fab2b2e5962314c2fca2f893246ce5cf4f

gtp/queue.h

@@ -23,7 +23,7 @@
 
 #define QUEUE_DEBUG 0		/* Print debug information */
 
-#define QUEUE_SIZE 1024		/* Size of retransmission queue */
+#define QUEUE_SIZE (PDP_MAX*2)	/* Size of retransmission queue */
 #define QUEUE_HASH_SIZE 65536	/* Size of hash table (2^16) */
 
 struct qmsg_t {			/* Holder for queued packets */