dma-pool: add pool sizes to debugfs

The atomic DMA pools can dynamically expand based on non-blocking
allocations that need to use it.

Export the sizes of each of these pools, in bytes, through debugfs for
measurement.

Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Rientjes <rientjes@google.com>
[hch: remove the !CONFIG_DEBUG_FS stubs]
Signed-off-by: Christoph Hellwig <hch@lst.de>