liquidio: Replace vmalloc + memset with vzalloc

Use vzalloc and vzalloc_node instead of using vmalloc and
vmalloc_node and then zeroing the allocated memory by
memset 0.
This simplifies the code.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>