use exact allocation for dma coherent memory

[Christoph Hellwig]

Hi all,

various architectures have used exact memory allocations for dma
allocations for a long time, but x86 and thus the common code based
on it kept using our normal power of two allocator, which tends to
waste a lot of memory for certain allocations.

Switching to a slightly cleaned up alloc_pages_exact is pretty easy,
but it turns out that because we didn't filter valid gfp_t flags
on the DMA allocator, a bunch of drivers were passing __GFP_COMP
to it, which is rather bogus in too many ways to explain.  Arm has
been filtering it for a while, but this series instead tries to fix
the drivers and warn when __GFP_COMP is passed, which makes it much
larger than just adding the functionality.