In case if there is the VMA which has a huge size (4,8,16 G)
then the prealoocated bitmap space won't be enough to track
all pages. Thus we call for system help invoking mmap() call
with bitmap length needed.

We simply assume that if such big VMA present the node has enough
memory to provide us space for pages huge bitmap.
Reported-by: Adrian Reber <adrian@lisas.de>
Tested-by: Adrian Reber <adrian@lisas.de>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>