[NETNS][IPV6] route6 - dynamically allocate ip6_dst_ops

ip6_dst_ops is dynamically allocated in init and exit functions.  That
provides the ability to do multiple instanciations of this structure.

This will be needed for network namespaces, indeed dst_ops stores data
that are required to be per namespace: entries and gc_thresh.

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>