Revert "sched/fair: Make update_min_vruntime() more readable"

There's a bug in this commit:

   97a7142f ("sched/fair: Make update_min_vruntime() more readable")

... when !rb_leftmost && curr we fail to advance min_vruntime.

So revert it.

Reported-by: Byungchul Park <byungchul.park@lge.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>