of: unittest: Remove VLA stack usage

The kernel would like to have all stack VLA usage removed[1].  This is a
test function so the execution speed is not critical.  We can allocate
memory for this buffer instead of using a VLA.  If kmalloc() fails just
return.

Allocate buffer with kmalloc().

[1]: https://lkml.org/lkml/2018/3/7/621



Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Rob Herring <robh@kernel.org>