It shouldn't fail due to OOM (it's boot time), and already warns if we
get two identical names.  But you never know what the future holds, and
WARN_ON_ONCE() keeps gcc happy with minimal code.
Reported-by: NLouis Langholtz <lou_langholtz@me.com>
Acked-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>