selftests: memfd_test: Revised STACK_SIZE to make it 16-byte aligned

There is a mandate of 16-byte aligned stack on AArch64 [1], so the
STACK_SIZE here should also be 16-byte aligned, otherwise we would
get an error when calling clone().

[1] http://lxr.free-electrons.com/source/arch/arm64/kernel/process.c#L265



Signed-off-by: Chunyan Zhang <zhang.chunyan@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>