selftests/xsk: display command line options with -h

Add the -h option to display all available command line options
available for test_xsk.sh and xskxceiver.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20230914084900.492-11-magnus.karlsson@gmail.com


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