params: suppress unused variable error, warn once just in case code changes.

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: Louis Langholtz <lou_langholtz@me.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>