vmem: fix pages purging for file mapped memory

madvise syscall with MADV_DONTNEED zero-fill-on-demand pages only for
mappings without an underlying file. In case of memory mapped to
underlying file pages will be reloaded with file's content.
Therefore we have to return valid unzeroed flag in pages_purge function,
depending on the memory type.
Actually it is not a size of block but number of words (integers).