[IPSEC]: Forbid BEET + ipcomp for now

While BEET can theoretically work with IPComp the current code can't
do that because it tries to construct a BEET mode tunnel type which
doesn't (and cannot) exist.  In fact as it is it won't even attach a
tunnel object at all for BEET which is bogus.

To support this fully we'd also need to change the policy checks on
input to recognise a plain tunnel as a legal variant of an optional
BEET transform.

This patch simply fails such constructions for now.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>