mm, compaction: properly signal and act upon lock and need_sched() contention

Compaction uses compact_checklock_irqsave() function to periodically check
for lock contention and need_resched() to either abort async compaction,
or to free the lock, schedule and retake the lock.  When aborting,
cc->contended is set to signal the contended state to the caller.  Two
problems have been identified in this mechanism.

First, compaction also calls directly cond_resched() in both scanners when
no lock is yet taken.  This call either does not abort async compaction,
or set cc->contended appropriately.  This patch introduces a new
compact_should_abort() function to achieve both.  In isolate_freepages(),
the check frequency is reduced to once by SWAP_CLUSTER_MAX pageblocks to
match what the migration scanner does in the preliminary page checks.  In
case a pageblock is found suitable for calling isolate_freepages_block(),
the checks within there are done on higher frequency.

Second, isolate_freepages() does not check ...