mm: compaction: iron out isolate_freepages_block() and isolate_freepages_range()

mm: compaction: iron out isolate_freepages_block() and isolate_freepages_range()

Andrew pointed out that isolate_freepages_block() is "straggly" and
isolate_freepages_range() is making assumptions on how compact_control is
used which is delicate.  This patch straightens isolate_freepages_block()
and makes it fly straight and initialses compact_control to zeros in
isolate_freepages_range().  The code should be easier to follow and is
functionally equivalent.  The CMA failure path is now a little more
expensive but that is a marginal corner-case.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Richard Davies <richard@arachsys.com>
Cc: Shaohua Li <shli@kernel.org>
Cc: Avi Kivity <avi@redhat.com>
Cc: Rafael Aquini <aquini@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>