cr-restore: Report the mem size if mmap failed

Without patch

 | Error (cr-restore.c:414): Unable to reserve memory: Invalid argument

with applied

 | Error (cr-restore.c:414): Unable to reserve memory (0 bytes): Invalid argument

more convenient for debugging.

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