sd-netlink: constify object pointers passed to getters

sd-netlink: constify object pointers passed to getters

sd-netlink is not public yet, so we can change the interface.

I did not touch interfaces of functions like sd_netlink_wait() and
sd_rtnl_message_new_link() which do not modify the object that is passed in,
because in the future we might want to change the code to e.g. take a
reference to the parent object or otherwise require a non-const reference.