selftests/xsk: add debug option for creating netdevs

selftests/xsk: add debug option for creating netdevs

Add a new option to the test_xsk.sh script that only creates the two
veth netdevs and the extra namespace, then exits without running any
tests. The failed test can then be executed in the debugger without
having to create the netdevs and namespace manually. For ease-of-use,
the veth netdevs to use are printed so they can be copied into the
debugger.

Here is an example how to use it:

> sudo ./test_xsk.sh -d

veth10 veth11

> gdb xskxceiver

In gdb:

run -i veth10 -i veth11

And now the test cases can be debugged with gdb.

If you want to debug the test suite on a real NIC in loopback mode,
there is no need to use this feature as you already know the netdev of
your NIC.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/r/20230111093526.11682-7-magnus.karlsson@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>