mm/memory_hotplug: Reduce migration re-try timeout value

Currently, the timeout value for migration re-tries during
memory offlining is 120 seconds. If migration encounters a
pinned page, it is guaranteed to fail no matter how many
re-tries are attempted, and there is much CPU cycles spent
on the re-tries with this timeout, which would contribute
to power. Hence, reduce the timeout to half, 60 seconds.

Change-Id: Ib300c4c1c3c4f5d10f5d8652d04a783c0f459fc1
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>