ipv6: restore the behavior of ipv6_sock_ac_drop()

It is possible that the interface is already gone after joining
the list of anycast on this interface as we don't hold a refcount
for the device, in this case we are safe to ignore the error.

What's more important, for API compatibility we should not
change this behavior for applications even if it were correct.

Fixes: commit a9ed4a29

 ("ipv6: fix rtnl locking in setsockopt for anycast and multicast")
Cc: Sabrina Dubroca <sd@queasysnail.net>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>