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.
Yes this occurs only on debug version as only this version has assertions.
I've already checked vmem_calloc() for this case and it was fine but I will add additional test for vmem_calloc.