[NETNS]: Place fib tables into netns.

The preparatory work has been done. All we need is to substitute
fib_table_hash with net->ipv4.fib_table_hash. Netns context is
available when required.

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>