[NETNS][IPV6] ip6_fib - dynamically allocate gc-timer

The ip6_fib_timer gc timer is dynamically allocated and initialized in
the ip6 fib init function. There are no more references to a static
global variable. That will allow to make multiple instance of the
garbage collecting timer and make them per namespace.

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>