fix dma coherent pool sizing

[Christoph Hellwig]

Hi all

Michal pointed out that the sizing of the dma coherent pools is bonkers
as it uses the total memory to size the pool for each zone, which leads
to comically larger zones for the 16-MB ZONE_DMA on x86, which tends
leads to allocation failure warnings.  This series switches to sizing
the ZONE_DMA and ZONE_DMA32 pools based on the number of pages that
actually reside in those zones instead.

Diffstat:
 pool.c |   56 +++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 33 insertions(+), 23 deletions(-)