Performance experiments show, that we spend (relatively) a lot of time
mremap-ing areas from premap area into their proper places. This time
depends on the task being restored, but for those with many vmas this
can be up to 20%.

The thing is that premapping is only needed to restore cow pages since
we don't have any API in the kernel to share a page between two or more
anonymous vmas. For non-cowing areas we map mmap() them directly in
place. But for such cases we'll also need to restore the page's contents
also from the pie code.

Doing the whole page-read code from PIE is way too complex (for now), so
the proposal is to optimize the case when we have a single local pagemap
layer. This is what pr.pieok boolean stands for.

v2:
* Fixed ARM compiling (vma addresses formatting)
* Unused tail of premapped area was left in task after restore
* Preadv-ing pages in restorer context worked on corrupted iovs
  due to mistakes in pointer arithmetics
* AIO mapping skipped at premap wasn't mapped in pie
* Growsdown VMAs should sometimes (when they are "guarded" by
  previous VMA and guard page's contents cannot be restored in
  place) be premmaped
* Always premmap for lazy-pages restore
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>