Revert "powerpc: Rework dma-noncoherent to use generic vmalloc layer"

This reverts commit 33f00dcedb0e22cdb156a23632814fc580fcfcf8.

    While it was a good idea to try to use the mm/vmalloc.c allocator instead
    of our own (in fact, ours is itself a dup on an old variant of the vmalloc
    one), unfortunately, the approach is terminally busted since
    dma_alloc_coherent() can be called at interrupt time or in atomic contexts
    and there's little chances we'll make the code in mm/vmalloc.c cope with\       that :-(

    Until we can get the generic code to forbid that idiocy and fix all
    drivers abusing it, we pretty much have no choice but revert to
    our custom virtual space allocator.

    There's also a problem with SMP safety since freeing such mapping
    would require an IPI which cannot be done at interrupt time.

    However, right now, I don't think we support any platform that is
    both SMP and has non-coherent DMA (don't laugh, I know such things
    do exist !) so we can sort that out later.

    Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

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