NVMe: Use kzalloc instead of kmalloc+memset

Use kzalloc instead of kmalloc and a susbsequent memset.

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Signed-off-by: Vishal Verma <vishal.l.verma@linux.intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>