selftests: xsk: Split worker thread

selftests: xsk: Split worker thread

Let's a have a separate Tx/Rx worker threads instead of a one common
thread packed with Tx/Rx specific checks.

Move mmap for umem buffer space and a switch_namespace() call to
thread_common_ops.

This also allows for a bunch of simplifactions that are the subject of
the next commits. The final result will be a code base that is much
easier to follow.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210329224316.17793-10-maciej.fijalkowski@intel.com