sgsn: Don't send XID reset after Detach Accept

Currently when a Detach Accept is received for an unknown TLLI (which
is in general the case afer the SGSN has requested the detachment),
an XID reset is sent to the BSS, causing a BSSGP Status message. This
happens in gsm0408_rcv_gmm.

This patch moves the corresponding call to gprs_llgmm_reset downwards
so that it is not being called in that case.

Addresses:
SGSN->BSS TLLI: 0xd75b91d9 SAPI: LLGMM, UI (DTAP) (GMM) Detach Request
BSS->SGSN TLLI: 0xd75b91d9 SAPI: LLGMM, UI (DTAP) (GMM) Detach Accept
SGSN->BSS TLLI: 0xd75b91d9 SAPI: LLGMM, U, XID (Reset, IOV-UI)
BSS->SGSN TLLI: ---        BSSGP STATUS (Unknown MS)

Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/gprs/gprs_gmm.c b/openbsc/src/gprs/gprs_gmm.c
index a2c30be..ac063af 100644
--- a/openbsc/src/gprs/gprs_gmm.c
+++ b/openbsc/src/gprs/gprs_gmm.c
@@ -1223,8 +1223,6 @@
 			return 0;
 		}
 
-		gprs_llgmm_reset(llme);
-
 		/* Don't reply or establish a LLME on DETACH_ACK */
 		if (gh->msg_type == GSM48_MT_GMM_DETACH_ACK) {
 			/* TLLI unassignment */
@@ -1232,6 +1230,8 @@
 						 GPRS_ALGO_GEA0, NULL);
 		}
 
+		gprs_llgmm_reset(llme);
+
 		/* Don't force it into re-attachment */
 		if (gh->msg_type == GSM48_MT_GMM_DETACH_REQ) {
 			/* Handle Detach Request */