[NETNS][IPV6] route6 - make garbage collection work with multiple network namespaces

This patch makes the necessary changes to make IPv6 dst_entry garbage
collection work with multiple network namespaces.

In ip6_dst_gc(), static local variables are now declared
per-namespace.

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