fib: add missing attribute validation for tun_id

Add missing netlink policy entry for FRA_TUN_ID.

Fixes: e7030878 ("fib: Add fib rule match on tunnel id")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>