selftests/xsk: add packet iterator for tx to packet stream

Convert the current variable rx_pkt_nb to an iterator that can be used
for both Rx and Tx. This to simplify the code and making Tx more like
Rx that already has this feature.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20230516103109.3066-6-magnus.karlsson@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>