fix #28339081

commit da83a722959a82733c3ca60030cc364ca2318c5a upstream

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: NFredrik Noring <noring@nocrew.org>
Reviewed-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NBaolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: NAlex Shi <alex.shi@linux.alibaba.com>