sched/numa: Avoid task migration for small numa improvement

If numa improvement from the task migration is going to be very
minimal, then avoid task migration.

specjbb2005 / bops/JVM / higher bops are better
on 2 Socket/2 Node Intel
JVMS  Prev    Current  %Change
4     200892  210118   4.59252
1     325766  313171   -3.86627

on 2 Socket/4 Node Power8 (PowerNV)
JVMS  Prev     Current  %Change
8     89011.9  91027.5  2.26442
1     211338   216460   2.42361

on 2 Socket/2 Node Power9 (PowerNV)
JVMS  Prev    Current  %Change
4     190261  191918   0.870909
1     195305  207043   6.01009

on 4 Socket/4 Node Power7
JVMS  Prev     Current  %Change
8     57651.1  58462.1  1.40674
1     111351   108334   -2.70945

dbench / transactions / higher numbers are better
on 2 Socket/2 Node Intel
count  Min      Max      Avg      Variance  %Change
5      12254.7  12331.9  12297.8  28.1846
5      11851.8  11937.3  11890.9  33.5169   -3.30872

on 2 Socket/4 Node Power8 (PowerNV)
count  Min      Max      Avg      Variance  %Change
5      4997.83  5030.14  5015.54  12.947
5      4791     5016.08  4962.55  85.9625   -1.05652

on 2 Socket/2 Node Power9 (PowerNV)
count  Min      Max      Avg      Variance  %Change
5      9331.84  9375.11  9352.04  16.0703
5      9353.43  9380.49  9369.6   9.04361   0.187767

on 4 Socket/4 Node Power7
count  Min      Max      Avg      Variance  %Change
5      147.55   181.605  168.963  11.3513
5      149.518  215.412  179.083  21.5903   5.98948

Cc: Rik van Riel <riel@surriel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1533276841-16341-4-git-send-email-srikar@linux.vnet.ibm.com