On real apps it's typical to have sequences ov VMAs with
absolutely the same file mapped. We've seen this dump-time
and fixed multiple openings of map_files links with the
file_borrowed flag.

Restore situation is the same -- the vm_open() call in many
cases re-open the same path with the same flags. This slows
things down.

To fix this -- chain VMAs with mapped files to each other
and only the first one opens the file and only the last
one closes it.

✓ travis-ci: success for mem: Do not re-open files for mappings when not required
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>