mm, compaction: do not count compact_stall if all zones skipped compaction

The compact_stall vmstat counter counts the number of allocations stalled
by direct compaction.  It does not count when all attempted zones had
deferred compaction, but it does count when all zones skipped compaction.
The skipping is decided based on very early check of
compaction_suitable(), based on watermarks and memory fragmentation.
Therefore it makes sense not to count skipped compactions as stalls.
Moreover, compact_success or compact_fail is also already not being
counted when compaction was skipped, so this patch changes the
compact_stall counting to match the other two.

Additionally, restructure __alloc_pages_direct_compact() code for better
readability.

Signed-off-by: Vlastimil Babka <[email protected]>
Cc: Minchan Kim <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Michal Nazarewicz <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Rik van Riel <[email protected]>
Acked-by: David Rientjes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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