staging: comedi: use dma_mmap_coherent for DMA-able buffer mmap

Comedi's acquisition buffer allocation code can allocate the buffer from
normal kernel memory or from DMA coherent memory depending on the
`dma_async_dir` value in the comedi subdevice.  (A value of `DMA_NONE`
causes the buffer to be allocated from normal kernel memory.  Other
values cause the buffer to be allocated from DMA coherent memory.)   The
buffer currently consists of a bunch of page-sized blocks that are
vmap'ed into a contiguous range of virtual addresses. The pages are also
mmap'able into user address space.  For DMA'able buffers, these
page-sized blocks are allocated by `dma_alloc_coherent()`.

For DMA-able buffers, the DMA API is currently abused in various ways,
the most serious abuse being the calling of `virt_to_page()` on the
blocks allocated by `dma_alloc_coherent()` and passing those pages to
`vmap()` (for mapping to the kernels vmalloc address space) and via
`page_to_pfn()` to `remap_pfn_range()` (for mmap'ing to user space).  it
also uses the `__GFP_COMP` flag when allocating the blocks, and marks
the allocated pages as reserved (which is unnecessary for DMA coherent
allocations).

The code can be changed to get rid of the vmap'ed address altogether if
necessary, since there are only a few places in the comedi code that use
the vmap'ed address directly and we also keep a list of the kernel
addresses for the individual pages prior to the vmap operation. This
would add some run-time overhead to buffer accesses.  The real killer is
the mmap operation.

For mmap, the address range specified in the VMA needs to be mmap'ed to
the individually allocated page-sized blocks.  That is not a problem
when the pages are allocated from normal kernel memory as the individual
pages can be remapped by `remap_pfn_range()`, but it is a problem when
the page-sized blocks are allocated by `dma_alloc_coherent()` because
the DMA API currently has no support for splitting a VMA across multiple
blocks of DMA coherent memory (or rather, no support for mapping part of
a VMA range to a single block of DMA coherent memory).

In order to comply with the DMA API and allow the buffer to be mmap'ed,
the buffer needs to be allocated as a single block by a single call to
`dma_alloc_coherent()`, freed by a single call to `dma_free_coherent()`,
and mmap'ed to user space by a single call to `dma_mmap_coherent()`.
This patch changes the buffer allocation, freeing, and mmap'ing code to
do that, with the unfortunate consequence that buffer allocation is more
likely to fail.  It also no longer uses the `__GFP_COMP` flag when
allocating DMA coherent memory, no longer marks the
allocated pages of DMA coherent memory as reserved, and no longer vmap's
the DMA coherent memory pages (since they are contiguous anyway).

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>