Revert "pagemap.h: Allow readahead Movable allocations from CMA region"

This reverts commit aae482fc ("pagemap.h: Allow readahead Movable
allocations from CMA region")

This change was added in order to improve the memory utilization of the
Movable zone in targets with large Movable zones.

Unfortunately it was found that adding file cache pages to the Movable
zone can sometimes significantly reduce the memory headroom of the device.
What can happen is that when the system gets under memory pressure it runs
kswapd, which uses a gfp mask of GFP_KERNEL. When the low memory killer
shrinker is called on behalf of kswapd with a gfp mask of GFP_KERNEL it
will ignore all free and reclaimable memory, such as the file cache pages,
that are located in the Movable zone. This can cause the low memory killer
to kill applications prematurely.

There are potential changes which could be made to optimize the system to
address the issue described above, however these changes are intrusive and
potentially risky.

Since we aren't currently running with a large Movable zone and because we
don't want to introduce any potentially risky changes this late in the
development cycle revert this change for now.

If in the future we want to re-visit improving the utilization of the
Movable zone we can pull this change back in and attempt to optimize the
memory reclaim framework to avoid the issues described above.

Change-Id: Icaeea4db60a8d538348a63b76b5e176ba69dfd0d
Signed-off-by: Liam Mark <lmark@codeaurora.org>