sched/fair: Add tmp_alone_branch assertion

The magic in list_add_leaf_cfs_rq() requires that at the end of
enqueue_task_fair():

  rq->tmp_alone_branch == &rq->lead_cfs_rq_list

If this is violated, list integrity is compromised for list entries
and the tmp_alone_branch pointer might dangle.

Also, reflow list_add_leaf_cfs_rq() while there. This looses one
indentation level and generates a form that's convenient for the next
patch.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>