All private vmas are placed in a premmapped region and
they are sorted by start addresses, so they should be shifted apart.

Here is one more problem with overlapped temporary and target regions,
mremap could not remap such cases directly, so for such cases a vma is
remapped away and then remapped on a target place.

v2: fix accoding with Pavel's comments
v3: add a huge comment with pictures
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>