hsr: implement dellink to clean up resources

hsr_link_ops implements ->newlink() but not ->dellink(),
which leads that resources not released after removing the device,
particularly the entries in self_node_db and node_db.

So add ->dellink() implementation to replace the priv_destructor.
This also makes the code slightly easier to understand.

Reported-by:  <syzbot+c6167ec3de7def23d1e8@syzkaller.appspotmail.com>
Cc: Arvid Brodin <arvid.brodin@alten.se>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>