selftests: xsk: Rename worker_* functions that are not thread entry points

Rename worker_* functions that are not thread entry points to
something else. This was confusing. Now only thread entry points are
worker_something.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210825093722.10219-9-magnus.karlsson@gmail.com