vrf: Move fib6_table into net_vrf

A later patch removes rt6i_table from rt6_info. Save the ipv6
table for a VRF in net_vrf. fib tables can not be deleted so
no reference counting or locking is required.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>