devlink: fix possible use-after-free and memory leaks in devlink_init()

The pernet operations structure for the subsystem must be registered
before registering the generic netlink family.

Make an unregister in case of unsuccessful registration.

Fixes: 687125b5

 ("devlink: split out core code")
Signed-off-by: Vasiliy Kovalev <kovalev@altlinux.org>
Link: https://lore.kernel.org/r/20240215203400.29976-1-kovalev@altlinux.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>