selftests: xsk: Add use_poll to ifobject

Add a use_poll option to the ifobject so that we do not need to use a
test specific if-statement in the test runner.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20210907071928.9750-9-magnus.karlsson@gmail.com