Replace dd call by an normal write call

Replace dd call by an normal write call

We would create a file with 0 padding to force it to a certain size.
This seems fairly ugly, in particular when we later want to use the
blob in other contexts. Also, since 9e0b115e0c, this shouldn't be necessary.
And if we're just copying bytes from a file we already have open, with no
padding, we might just as well do this without forking.