[grps] Fix GTP data_ind call back message creation

We'll need to push data both before and after, so make sure there
is both headroom & tailroom

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/openbsc/src/gprs/sgsn_libgtp.c b/openbsc/src/gprs/sgsn_libgtp.c
index a28790a..b5560db 100644
--- a/openbsc/src/gprs/sgsn_libgtp.c
+++ b/openbsc/src/gprs/sgsn_libgtp.c
@@ -373,7 +373,7 @@
 	}
 	mm = pdp->mm;
 
-	msg = msgb_alloc_headroom(len+128, 128, "GTP->SNDCP");
+	msg = msgb_alloc_headroom(len+256, 128, "GTP->SNDCP");
 	ud = msgb_put(msg, len);
 	memcpy(ud, packet, len);