selftests: xsk: Introduce rx_on and tx_on in ifobject

Introduce rx_on and tx_on in the ifobject so that we can describe if
the thread should create a socket with only tx, rx, or both. This
eliminates some test specific if statements from the code. We can also
eliminate the flow vector structure now as this is fully specified
by the tx_on and rx_on variables.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20210907071928.9750-10-magnus.karlsson@gmail.com