sched: Rename a misleading variable in build_overlap_sched_groups()

The child variable in build_overlap_sched_groups() actually refers to the
peer or sibling domain of the given CPU. Rename it to sibling to be consistent
with the naming in build_group_mask().

Signed-off-by: Zhihui Zhang <zzhsuny@gmail.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/1406942283-18249-1-git-send-email-zzhsuny@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>