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.h

@@ -5,6 +5,7 @@
 
 typedef enum SwitchRootFlags {
         SWITCH_ROOT_DESTROY_OLD_ROOT = 1 << 0, /* rm -rf old root when switching – under the condition that it is backed by non-persistent tmpfs/ramfs/… */
+        SWITCH_ROOT_DONT_SYNC        = 1 << 1, /* don't call sync() immediately before switching root */
 } SwitchRootFlags;
 
 int switch_root(const char *new_root, const char *old_root_after, SwitchRootFlags flags);