The config file contains a partial kernel configuration to be used by
`virtme-configkernel --custom'. The presumption is that the config file
contains all Kconfig options needed by the selftests from the directory.

In net/forwarding/config, many are missing, which manifests as spurious
failures when running the selftests, with messages about unknown device
types, qdisc kinds or classifier actions. Add the missing configurations.

Tested the resulting configuration using virtme-ng as follows:

 # vng -b -f tools/testing/selftests/net/forwarding/config
 # vng --user root
 (within the VM:)
 # make -C tools/testing/selftests TARGETS=net/forwarding run_tests
Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/025abded7ff9cea5874a7fe35dcd3fd41bf5e6ac.1706286755.git.petrm@nvidia.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>