mount: write drop-in file when setting transient properties

This fixes transient mount units in general. With this change,
'from_fragment' is for transient mount units. As a result, the normal logic
for 'fragment' mount units vs. 'mountinfo'  mount units works for transient
mount units as well.

src/core/dbus-mount.c

@@ -157,6 +157,9 @@ static int bus_mount_set_transient_property(
                 if (!p)
                         return -ENOMEM;
 
+                unit_write_drop_in_format(UNIT(m), mode, name, "[Mount]\n%s=%s\n",
+                        name, new_property);
+
                 free(*property);
                 *property = p;
         }