add osmo_gsup_msgb_alloc()

Throughout osmo-hlr's code, the GSUP msgb allocation is duplicated as:

  msgb_alloc_headroom(1024+16, 16, "foo");

Instead, use one common function to keep the magic numbers in one place.

Change-Id: I40e99b5bc4fd8f750da7643c03b2119ac3bfd95e
diff --git a/src/luop.c b/src/luop.c
index 652ce0d..1a2f854 100644
--- a/src/luop.c
+++ b/src/luop.c
@@ -50,8 +50,7 @@
 {
 	struct msgb *msg_out;
 
-	msg_out = msgb_alloc_headroom(1024+16, 16, "GSUP LUOP");
-	OSMO_ASSERT(msg_out);
+	msg_out = osmo_gsup_msgb_alloc("GSUP LUOP");
 	osmo_gsup_encode(msg_out, gsup);
 
 	osmo_gsup_addr_send(luop->gsup_server, luop->peer,