net: rename rtnl_net_debug to lock_debug

And make it selected by CONFIG_DEBUG_NET. Don't rename any of
the structs/functions. Next patch will use rtnl_net_debug_event in
netdevsim.

Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250401163452.622454-5-sdf@fomichev.me


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