selftests: net: Refactor cleanup logic in lib_netcons.sh

selftests: net: Refactor cleanup logic in lib_netcons.sh

Extract the network device and namespace cleanup logic from the
cleanup() function into a new do_cleanup() helper in lib_netcons.sh.

The do_cleanup() function only unconfigure the network and
printk, while cleanup() cleans the netconsole targets plus the network
and printk.

This refactoring let this code to be reused in cases netconsole dynamic
is not being used, as in the upcoming patch.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250613-rework-v3-7-0752bf2e6912@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>