mm: cma: on movable allocations try MIGRATE_CMA first

It has been observed that system tends to keep a lot of CMA free pages
even in very high memory pressure use cases.  The CMA fallback for
movable pages is used very rarely, only when system is completely
pruned from MOVABLE pages.  This means that the out-of-memory is
triggered for unmovable allocations even when there are many CMA pages
available.  This problem was not observed previously since movable
pages were used as a fallback for unmovable allocations.

To avoid such situation this commit changes the allocation order so
that on movable allocations the MIGRATE_CMA pageblocks are used first.

This change means that the MIGRATE_CMA can be removed from fallback
path of the MIGRATE_MOVABLE type.  This means that the
__rmqueue_fallback() function will never deal with CMA pages and thus
all the checks around MIGRATE_CMA can be removed from that function.

Change-Id: Ie13312d62a6af12d7aa78b4283ed25535a6d49fd
CRs-Fixed: 435287
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>