[IPV6]: Fix rtnetlink dump infinite loop

The recent change to netlink dump "done" callback handling broke IPv6
which played dirty tricks with the "done" callback.  This causes an
infinite loop during a dump.

The following patch fixes it.

This bug was reported by Jeff Garzik.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>