ARM: dma-api: fix off-by-one error in __dma_supported()
When validating the mask against the amount of memory we have available
(so that we can trap 32-bit DMA addresses with >32-bits memory), we had
not taken account of the fact that max_pfn is the maximum PFN number
plus one that would be in the system.
There are several references in the code which bear this out: