pagema-cache: Don't exceed task size on hight bound

If area is close to TASK_SIZE limit the high address
may exceed it returning error once we try to read
pagemap file. So limit the high address.

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