- Use "mkdir -p" to avoid errors when intermediate directories are
missing.
- Fall back to "dd" when "fallocate" fails. For example, fallocate isn't
supported on ext4.
- Add error checking for test image generation. Without this, the test
simply plows on spewing all kinds of errors which are hard to
immediately root-cause.
Signed-off-by: Stephen Warren <swarren@nvidia.com>