[NETNS]: Add netns to nl_info structure.

nl_info is used to track the end-user destination of routing change
notification. This is a natural object to hold a namespace on. Place
it there and utilize the context in the appropriate places.

Acked-by: Benjamin Thery <benjamin.thery@bull.net>
Acked-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>