switch-root: disable sync() again when we switch root during shutdown

Our shutdown binary that takes over as PID 1 when shutting down puts
great efforts into a sync() that comes with a time-out once sync'ing
process stops. If we'd add another dumb sync() here, we kinda defeat all
it is good for. Hence, let's keep the sync() in for most codepats, but
let's disable it for the final shutdown logic when we transition back
into the exitrd. After all we sync()ed more than enough here, no need to
sync() even more.

src/shared/switch-root.c

@@ -89,7 +89,8 @@ int switch_root(const char *new_root,
          * all while making them invisible/inaccessible in the file system tree for later code. That makes
          * sync'ing them then difficult. Let's hence issue a manual sync() here, so that we at least can
          * guarantee all file systems are an a good state before entering this state. */
-        sync();
+        if (!FLAGS_SET(flags, SWITCH_ROOT_DONT_SYNC))
+                sync();
 
         /* Work-around for kernel design: the kernel refuses MS_MOVE if any file systems are mounted
          * MS_SHARED. Hence remount them MS_PRIVATE here as a work-around.