gmm: mmctx_timer_stop(): warn about timer not running

This turns errors like:

  DMM ERROR MM(262420000000038/e2ff704e) Stopping MM timer 3350 but 0 is running

into warnings with a more accurate reason:

  DMM NOTICE MM(262420000000037/e2ff704e) Stopping *inactive* MM timer 3350

Change-Id: I56ecad9d8f1049974b0896f6d0e7fc61580155ec
---
M src/sgsn/gprs_gmm.c
1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/src/sgsn/gprs_gmm.c b/src/sgsn/gprs_gmm.c
index 768b321..653c425 100644
--- a/src/sgsn/gprs_gmm.c
+++ b/src/sgsn/gprs_gmm.c
@@ -111,9 +111,14 @@
 
 static void mmctx_timer_stop(struct sgsn_mm_ctx *mm, unsigned int T)
 {
-	if (mm->T != T)
+	if (!osmo_timer_pending(&mm->timer)) {
+		LOGMMCTXP(LOGL_NOTICE, mm, "Stopping *inactive* MM timer %u\n", T);
+		return;
+	}
+	if (mm->T != T) {
 		LOGMMCTXP(LOGL_ERROR, mm, "Stopping MM timer %u but "
 			"%u is running\n", T, mm->T);
+	}
 	osmo_timer_del(&mm->timer);
 }
 

