[CRIU] [RFC] [PATCH cr 0/16] Restore copy-on-write regions (v3)

[Andrey Vagin]

This series is based on two facts.
* Copy-on-write regions can be in MAP_PRIVATE mappings
* pages-[PID].img is a content of MAP_PRIVATE mappings

So we can restore all private mappings before forking children and
all copy-on-write regions will be restored automatically.

Here is only one problem, that a mapping can not be restored in a
correct place, because it may be conflict with crtools' mappings.
The mapping get avaliable address and then remmaped in restorer.

v2: * Remove unshared pages from inherited mappings.
    * Avoid conflict of premmapped vma-s and target vma-s
v3: * simplify code and add more comments
    * restore: don't add unneeded vma with zero start and end addresses