selftests/xsk: add option to only run tests in a single mode

Add an option -m on the command line that allows the user to run the
tests in a single mode instead of all of them. Valid modes are skb,
drv, and zc (zero-copy). An example:

To run test suite in drv mode only:

./test_xsk.sh -m drv

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20230914084900.492-4-magnus.karlsson@gmail.com


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