ipv6: fix a refcnt leak with peer addr

There is no reason to take a refcnt before deleting the peer address route.
It's done some lines below for the local prefix route because
inet6_ifa_finish_destroy() will release it at the end.
For the peer address route, we want to free it right now.

This bug has been introduced by commit
caeaba79

 ("ipv6: add support of peer address").

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>