[CRIU] Can't restore a process, which has been dumped in a second time

[Andrey Vagin]

The problem is that the restorer can't mmap vma with zero size.
I found that this vma is a part of stack from a first "crtools restore".

We already tried to fix such problem by df81acafa711811208442059e193f239c8f98c8d
"""
restorer: Calculate self-stack size with guard pages

Otherwise we might not unmap the whole stack leaving
a guard page in restoree memory map. Not a big deal
but anyway.

Signed-off-by: Cyrill Gorcunov <gorcunov at openvz.org>
Signed-off-by: Pavel Emelyanov <xemul at parallels.com>
"""

We care about own stack, but don't care about a previous stacks
(e.g:before clone(...)).

I think we should find a way to detect the flag MAP_GROWSDOWN... Do
you have any ideas?