mtd: tests: consolidate kmalloc/memset 0 call to kzalloc

This is an API consolidation only. The use of kmalloc + memset to 0
is equivalent to kzalloc.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>