engine/mt: Ensure master lock held for reload

Issue: 7819

DetectEngineReload must hold the `master->lock`; recent changes changed
the locking usages to avoid deadlock when registering/handling tenants.
These changes added the presumption that the master lock is held at a
higher level. Coverity highlighted that the lock is not held
consistently.

src/detect-engine.c

@@ -2303,6 +2303,7 @@ static void InjectPackets(
  *
  *  If called in unix socket mode, it's possible that we don't have
  *  detect threads yet.
+ *  NOTE: master MUST be locked before calling this
  *
  *  \retval -1 error
  *  \retval 0 no detection threads
@@ -4859,8 +4860,13 @@ int DetectEngineReload(const SCInstance *suri)
     DetectEngineDeReference(&old_de_ctx);
 
     SCLogDebug("going to reload the threads to use new_de_ctx %p", new_de_ctx);
+
+    DetectEngineMasterCtx *master = &g_master_de_ctx;
+    SCMutexLock(&master->lock);
     /* update the threads */
     DetectEngineReloadThreads(new_de_ctx);
+    SCMutexUnlock(&master->lock);
+
     SCLogDebug("threads now run new_de_ctx %p", new_de_ctx);
 
     /* walk free list, freeing the old_de_ctx */