ip6_vti: use IS_REACHABLE to avoid some compile errors

Naresh reported some compile errors:

  arm build failed due this error on linux-next 20200713 and  20200713
  net/ipv6/ip6_vti.o: In function `vti6_rcv_tunnel':
  ip6_vti.c:(.text+0x1d20): undefined reference to `xfrm6_tunnel_spi_lookup'

This happened when set CONFIG_IPV6_VTI=y and CONFIG_INET6_TUNNEL=m.
We don't really want ip6_vti to depend inet6_tunnel completely, but
only to disable the tunnel code when inet6_tunnel is not seen.

So instead of adding "select INET6_TUNNEL" for IPV6_VTI, this patch
is only to change to IS_REACHABLE to avoid these compile error.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Fixes: 08622869

 ("ip6_vti: support IP6IP6 tunnel processing with .cb_handler")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>