[NETLINK]: Kill CB only when socket is unused

Since we can still receive packets until all references to the
socket are gone, we don't need to kill the CB until that happens.
This also aligns ourselves with the receive queue purging which
happens at that point.

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