Merge branch 'selftests-mlxsw-convert-to-iproute2-dcb'

Petr Machata says:

====================
selftests: mlxsw: Convert to iproute2 dcb

There is a dedicated tool for configuration of DCB in iproute2. Use it
in the selftests instead of lldpad.

Patches #1-#3 convert three tests. Patch #4 drops the now-unnecessary
lldpad helpers.
====================

Link: https://lore.kernel.org/r/cover.1675096231.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>