selftests: xsk: Validate tx stats on tx thread

Validate the tx stats on the Tx thread instead of the Rx
thread. Depending on your settings, you might not be allowed to query
the statistics of a socket you do not own, so better to do this on the
correct thread to start with.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210825093722.10219-11-magnus.karlsson@gmail.com