gre: reject GUE and FOU in collect metadata mode

gre: reject GUE and FOU in collect metadata mode

The collect metadata mode does not support GUE nor FOU. This might be
implemented later; until then, we should reject such config.

I think this is okay to be changed. It's unlikely anyone has such
configuration (as it doesn't work anyway) and we may need a way to
distinguish whether it's supported or not by the kernel later.

For backwards compatibility with iproute2, it's not possible to just check
the attribute presence (iproute2 always includes the attribute), the actual
value has to be checked, too.

Fixes: 2e15ea390e6f4 ("ip_gre: Add support to collect tunnel metadata.")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>