bus_dmamem_alloc() can't handle large NOWAIT requests

I've just been tracking down a panic in 4.x and found that the problem
is still present in 6.x.

According to bus_dma(9), bus_dmamem_alloc() can be invoked with a
flag BUS_DMA_NOWAIT to indicate that sleep()ing is not allowed.

At least on the i386, if the requested size exceeds 1 page (or some
other cases), the requested memory will be allocated via contigmalloc().

bus_dmamem_alloc() maps BUS_DMA_NOWAIT to M_NOWAIT but contigmalloc()
does not support M_NOWAIT and will tsleep() under some conditions.

Unfortunately, I don't know enough about the VM code to be able to
suggest a fix.

-- 
Peter Jeremy