vmem: fix pages purging for file mapped memory

Madvise system call with MADV_DONTNEED does _not_
fill pages with zeroes when memory is mapped to underlying file.
See madvice(2) for details.

pages_purge() function was saying that memory region has been zeroed
after calling madvice with MADV_DONTNEED - which was _not_ true.
This was causing an assertion fault because the page's meta-data was
invalid.

To fix this issue:
Return proper value in pages_purge() in order to set proper flag in
internal metadata. For file mapped memory pages_purge() does not
zero-fill pages. Only pool with id number 0 is anonymously mapped - pages are
filled with zeros.
Merge pull request #2 from plebioda/fix-unzeroed-pages-assert

vmem: fix pages purging for file mapped memory