sched/balancing: Prevent the reselection of a previous env.dst_cpu if some tasks...

sched/balancing: Prevent the reselection of a previous env.dst_cpu if some tasks are pinned

Currently new_dst_cpu is prevented from being reselected actually, not
dst_cpu. This can result in attempting to pull tasks to this_cpu twice.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/281f59b6e596c718dd565ad267fc38f5b8e5c995.1379265590.git.vdavydov@parallels.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>