selftests/xsk: merge dual and single thread dispatchers

Make the thread dispatching code common by unifying the dual and
single thread dispatcher code. This so we do not have to add code in
two places in upcoming commits.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/r/20230111093526.11682-14-magnus.karlsson@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>