mm/migrate: Use trylock while resetting rate limit

Since this spinlock will only serialize migrate rate limiting,
convert the spinlock to a trylock. If another task races ahead of this task
then this task can simply move on.

While here, add correct two abnormalities.
- Avoid time being stretched for every interval.
- Use READ/WRITE_ONCE with next window.

specjbb2005 / bops/JVM / higher bops are better
on 2 Socket/2 Node Intel
JVMS  Prev    Current  %Change
4     206350  200892   -2.64502
1     319963  325766   1.81365

on 2 Socket/2 Node Power9 (PowerNV)
JVMS  Prev    Current  %Change
4     186539  190261   1.99529
1     220344  195305   -11.3636

on 4 Socket/4 Node Power7
JVMS  Prev    Current  %Change
8     56836   57651.1  1.43413
1     112970  111351   -1.43312

dbench / transactions / higher numbers are better
on 2 Socket/2 Node Intel
count  Min      Max      Avg      Variance  %Change
5      13136.1  13170.2  13150.2  14.7482
5      12254.7  12331.9  12297.8  28.1846   -6.48203

on 2 Socket/4 Node Power8 (PowerNV)
count  Min      Max      Avg      Variance  %Change
5      4319.79  4998.19  4836.53  261.109
5      4997.83  5030.14  5015.54  12.947    3.70121

on 2 Socket/2 Node Power9 (PowerNV)
count  Min      Max      Avg      Variance  %Change
5      9325.56  9402.7   9362.49  25.9638
5      9331.84  9375.11  9352.04  16.0703   -0.111616

on 4 Socket/4 Node Power7
count  Min      Max      Avg      Variance  %Change
5      132.581  191.072  170.554  21.6444
5      147.55   181.605  168.963  11.3513   -0.932842

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>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1533276841-16341-3-git-send-email-srikar@linux.vnet.ibm.com