Use dma_zalloc_coherent for allocating zeroed
memory and remove unnecessary memset function.

Done using Coccinelle.
Generated-by: scripts/coccinelle/api/alloc/kzalloc-simple.cocci
0-day tested with no failures.
Suggested-by: NLuis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: NHimanshu Jha <himanshujha199640@gmail.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>