ip_tunnel: Provide tunnel metadata API for CONFIG_INET=n

Account for the configuration FIB_RULES=y && INET=n as FIB_RULES can
be selected by IPV6 or DECNET without INET.

Fixes: e7030878 ("fib: Add fib rule match on tunnel id")
Fixes: 3093fbe7 ("route: Per route IP tunnel metadata via lightweight tunnel")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>