If we're using 'hole-detection=raw' then we're going to be manually
looking for sparseness in our files. We don't really care about
whether the OS thinks they're sparse, so don't do the check about the
number of blocks the OS thinks the file takes.

Without this change if I use "tar --hole-detection=raw -cf" to tar up
a file that's got lots of zeros (but isn't marked by the OS as
sparse), when I untar the file then it's still not sparse. With this
change then the zeros are correctly transformed into sparseness.