When I did some header cleanup in r320528, I copy-pasted some sketchy WITNESS
checks from the prior busdma wrappers. Two of these were removed in r346351
and r346851, but one remains in bounce_bus_dmamem_alloc(). This check could
be constrained to only apply if (flags & BUS_DMA_NOWAIT), but it's cleaner to
simply remove it and rely on the checks already present in the sleepable
allocation paths used by this function.
Noticed by: cem