udev-extraconf:mount.sh: fix a umount issue

Only touching /tmp/.automount-$name is not good enough, it must contain
the mount name, otherwise umount could not get the path from it.

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>