cleanup vmap usage in the dma-mapping layer

[Christoph Hellwig]

Hi all,

the common DMA remapping code uses the vmalloc/vmap code to create
page table entries for DMA mappings.  This series lifts the currently
arm specific VM_* flag for that into common code, and also exposes
it to userspace in procfs to better understand the mappings, and cleans
up a couple helpers in this area.