pagema-cache: Don't exceed task size on hight bound
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 <7a1ea01eee6961eb1e372e3508c2670446d086f4@openvz.org>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

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