selftests: xsk: Remove struct ifaceconfigobj

ifaceconfigobj is not really useful, it is possible to keep the
functionality and simplify the code.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210329224316.17793-3-maciej.fijalkowski@intel.com