dma-mapping-fast: Align memory allocation to dma_alloc_attrs expectations

On newer kernels, memory allocated by dma_alloc_attrs() is
expected to be zeroed out upon allocation. The allocation
paths in fastmap do not currently account for this new
requirement, so update them to zero out memory, unless
the client explicitly asks for memory to not be zeroed out.

Change-Id: I5c7fa81ab4b8fa98890036082883769ab12443c6
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>