GPRS: enhance msgb headroom/tailroom for SGSN (GMM)

In the GPRS NS protocol stack, the amount of NS/BSSGP headers like MS RADIO
CAPA INFO can be quite long.  In order to fit the full user message and
those headers, we have to enlarge the head/tailroom of the msgb allocations.
diff --git a/openbsc/include/openbsc/gsm_04_08.h b/openbsc/include/openbsc/gsm_04_08.h
index 15ac1b3..fe88d1d 100644
--- a/openbsc/include/openbsc/gsm_04_08.h
+++ b/openbsc/include/openbsc/gsm_04_08.h
@@ -13,8 +13,8 @@
 struct gsm_trans;
 struct gsm_subscriber_connection;
 
-#define GSM48_ALLOC_SIZE	1024
-#define GSM48_ALLOC_HEADROOM	128
+#define GSM48_ALLOC_SIZE	2048
+#define GSM48_ALLOC_HEADROOM	256
 
 static inline struct msgb *gsm48_msgb_alloc(void)
 {