selftests: xsk: introduce validation functions

Introduce validation functions that can be optionally called by the Rx
and Tx threads. These are then used to replace the Rx and Tx stats
dispatchers. This so that we in the next commit can make the stats
tests proper normal tests and not be some special case, as today.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20220510115604.8717-8-magnus.karlsson@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>