mm, compaction: remove redundant watermark check in compact_finished()

When detecting whether compaction has succeeded in forming a high-order
page, __compact_finished() employs a watermark check, followed by an own
search for a suitable page in the freelists.  This is not ideal for two
reasons:

 - The watermark check also searches high-order freelists, but has a
   less strict criteria wrt fallback. It's therefore redundant and waste
   of cycles. This was different in the past when high-order watermark
   check attempted to apply reserves to high-order pages.

 - The watermark check might actually fail due to lack of order-0 pages.
   Compaction can't help with that, so there's no point in continuing
   because of that. It's possible that high-order page still exists and
   it terminates.

This patch therefore removes the watermark check.  This should save some
cycles and terminate compaction sooner in some cases.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Vlastimil Babka <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: David Rientjes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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