[NETNS][IPV6] route6 - Pass the network namespace parameter to rt6_purge_dflt_routers

Add a network namespace parameter to rt6_purge_dflt_routers.  This is
needed to call fib6_get_table with the appropriate network namespace.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>