netconsole: Add automatic console unregistration on target removal

Add unregister_netcons_consoles() function to automatically unregister
console handlers when no targets of the corresponding type remain active.

The function iterates through the target list to determine which console
types (basic vs extended) are still needed, and unregisters any console
handlers that are no longer required. This prevents having registered
console handlers without corresponding active targets.

The function is called when a target is disabled and moved to the cleanup
list, ensuring proper cleanup of unused console registrations.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250609-netcons_ext-v3-2-5336fa670326@debian.org


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