mm: track actual nr_scanned during shrink_slab()

Some shrinkers may only be able to free a bunch of objects at a time,
and so free more than the requested nr_to_scan in one pass.

Whilst other shrinkers may find themselves even unable to scan as many
objects as they counted, and so underreport.  Account for the extra
freed/scanned objects against the total number of objects we intend to
scan, otherwise we may end up penalising the slab far more than
intended.  Similarly, we want to add the underperforming scan to the
deferred pass so that we try harder and harder in future passes.

Link: http://lkml.kernel.org/r/20170822135325.9191-1-chris@chris-wilson.co.uk
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Shaohua Li <shli@fb.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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