netfiler: ipset: Add net namespace for ipset

This patch adds netns support for ipset.

Major changes were made in ip_set_core.c and ip_set.h.
Global variables are moved to per net namespace.
Added initialization code and the destruction of the network namespace ipset subsystem.
In the prototypes of public functions ip_set_* added parameter "struct net*".

The remaining corrections related to the change prototypes of public functions ip_set_*.

The patch for git://git.netfilter.org/ipset.git commit 6a4ec96c0b8caac5c35474e40e319704d92ca347

Signed-off-by: Vitaly Lavrov <lve@guap.ru>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>