umount-all: Add udev_settle after unmounting disks.

This helps avoid an error on Ubuntu, but it's not clear if this
is a real solution or just helps by adjusting the timing of some
race condition.

daemon/mount.c

@@ -324,6 +324,9 @@ do_umount_all (void)
 
   free_stringslen (mounts, size);
 
+  /* Without this, lvm_remove_all was failing on Ubuntu. */
+  udev_settle ();
+
   /* We've unmounted root now, so ... */
   root_mounted = 0;