For `criu exec` we are searching for a place for syscall injection.
While searching for a VMA with PROT_EXEC and with needed size,
we check that VMA is lower than task_size.
The callpath for it is:
cr_exec => parasite_prep_ctl => get_vma_by_ip

Firstly, I thought to omit kdat.task_size checking if it's not inited:
> if (vma_area->e->start >= kdat.task_size && kdat.task_size)
but I think it's a hack then a proper solution.
Besides, this code still can choose VMA over task_size on ARM
and try to inject syscall there (IIRC, ARM has kernel-mapped
VMA in that area).

So, lets init kdat.task_size for `criu exec`.
Also lets init kdat.has_compat_sigreturn so we could exec into
compatible applications.

Cc: Christopher Covington <cov@codeaurora.org>
Cc: Andrew Vagin <avagin@virtuozzo.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Reviewed-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>