selftests: net: lib: Rename ip_link_set_down() to adf_*

Rename this function to mark it as autodefer.
For details, see the discussion in the cover letter.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/e5bf4cb3405fb50fe6e217a04268952e97410dc2.1758821127.git.petrm@nvidia.com


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