IuPS: sgsn_mm_ctx: add enum gprs_pmm_state field, track PMM state

Iu needs to page to transfer data in PMM-IDLE state.

Change-Id: Id37778cb9a0328a21c8e8246998ecdb43dd687d8
diff --git a/openbsc/src/gprs/gprs_sgsn.c b/openbsc/src/gprs/gprs_sgsn.c
index dd7e008..e5a54d9 100644
--- a/openbsc/src/gprs/gprs_sgsn.c
+++ b/openbsc/src/gprs/gprs_sgsn.c
@@ -246,6 +246,7 @@
 	ctx->iu.ue_ctx = uectx;
 	ctx->iu.new_key = 1;
 	ctx->mm_state = GMM_DEREGISTERED;
+	ctx->pmm_state = PMM_DETACHED;
 	ctx->auth_triplet.key_seq = GSM_KEY_SEQ_INVAL;
 	ctx->ctrg = rate_ctr_group_alloc(ctx, &mmctx_ctrg_desc, 0);