mem-offline: improve the effective utilization of movable zone

When a memory block in a movable zone is onlined, only newly allocated
or swap in anon pages will fill it. This means that the migrated pages
from these memory blocks during offlining will sit in the lower zones
despite enough free space in the movable zone thus leaving pressure in
the lower zones. This pressure can effect the performance for the
allocations which can only be satisfied from lower zones.
This is avoided by filling in eligible pages from normal zone to movable
zone thus results in effective utilization of movable zone and also free
up memory in the lower zones.

Change-Id: I3404b9078e942c98f497cb462bc24fc53fb075ba
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>