test/fs: error case fixes/enhancements

- 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>