ms: Rename field timer -> release_timer

Rename it so that it becomes clear what the timer is used for.

Change-Id: Id4738ea196b232858e1ec9cdc439f9b81839058f
diff --git a/src/gprs_ms.h b/src/gprs_ms.h
index 4f6456e..b2b3274 100644
--- a/src/gprs_ms.h
+++ b/src/gprs_ms.h
@@ -79,7 +79,7 @@
 	struct osmo_timer_list llc_timer;
 
 	struct osmo_use_count use_count;
-	struct osmo_timer_list timer;
+	struct osmo_timer_list release_timer;
 
 	int64_t last_cs_not_low;