selftests: xsk: Remove mutex and condition variable

The usage of the condition variable is broken, and overkill. Replace it
with a pthread barrier.

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210329224316.17793-17-maciej.fijalkowski@intel.com