lib/genalloc: add gen_pool_dma_zalloc() for zeroed DMA allocations

gen_pool_dma_zalloc() is a zeroed memory variant of
gen_pool_dma_alloc().  Also document the return values of both, and
indicate NULL as a "%NULL" constant.

Signed-off-by: Fredrik Noring <noring@nocrew.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>