[NETNS][IPV6] ip6_fib - gc timer per namespace

Move the timer initialization at the network namespace creation and
store the network namespace in the timer argument.

That enables multiple timers (one per network namespace) to do garbage
collecting.

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