ipv6: delete protocol and unregister rtnetlink when cleanup

pim6_protocol was added when initiation, but it not deleted.
Similarly, unregister RTNL_FAMILY_IP6MR rtnetlink.

Signed-off-by: Duan Jiong <duanj.fnst@cn.fujitsu.com>
Reviewed-by: Cong Wang <cwang@twopensource.com>
Signed-off-by: David S. Miller <davem@davemloft.net>