selftests: rtnetlink: print device info on preferred_lft test failure

Even with slowwait used to avoid system sleep in the preferred_lft test,
failures can still occur after long runtimes.

Print the device address info when the test fails to provide better
troubleshooting data.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20250819074749.388064-1-liuhangbin@gmail.com


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