[NETFILTER] Inherit masq_index to slave connections

masq_index is used for cleanup in case the interface address changes
(such as a dialup ppp link with dynamic addreses).  Without this patch,
slave connections are not evicted in such a case, since they don't inherit
masq_index.

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>