[NETNS][IPV6]: Make ip6_frags per namespace.

The ip6_frags is moved to the network namespace structure.  Because
there can be multiple instances of the network namespaces, and the
ip6_frags is no longer a global static variable, a helper function has
been added to facilitate the initialization of the variables.

Until the ipv6 protocol is not per namespace, the variables are
accessed relatively from the initial network namespace.

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