IB: Remove unneeded memset

In commit af7ddd8a ("Merge tag 'dma-mapping-4.21' of
git://git.infradead.org/users/hch/dma-mapping"

),
dma_alloc_coherent/dmam_alloc_coherent always zeroed the returned memory.
So the memset after a coherent allocation function is not needed.

Signed-off-by: Fuqian Huang <huangfq.daxian@gmail.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>