ipv6 netns: Address labels per namespace

This pacth makes IPv6 address labels per network namespace.
It keeps the global label tables, ip6addrlbl_table, but
adds a 'net' member to each ip6addrlbl_entry.
This new member is taken into account when matching labels.

Changelog
=========
* v1: Initial version
* v2:
  * Minize the penalty when network namespaces are not configured:
      *  the 'net' member is added only if CONFIG_NET_NS is
         defined. This saves space when network namespaces are not
         configured.
      * 'net' value is retrieved with the inlined function
         ip6addrlbl_net() that always return &init_net when
         CONFIG_NET_NS is not defined.
  * 'net' member in ip6addrlbl_entry renamed to the less generic
    'lbl_net' name (helps code search).

Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>