mtd-tests: io_paral: build error, "variable length strings"

mtd-tests: io_paral: build error, "variable length strings"

Some compilers will complain about use of strlen() within a static array
size declaration. For this type of string, "sizeof() - 1" is equivalent
and prevents the build error.

Error:

   io_paral.c:48:13: error: variably modified 'vol_nodes' at file scope

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <dedekind1@gmail.com>