Revert "sched/fair: Move unused stub functions to header"

Revert commit 7aa55f2a ("sched/fair: Move unused stub functions to
header"), for while it has the right Changelog, the actual patch
content a revert of the previous 4 patches:

  f7df852a ("sched: Make task_vruntime_update() prototype visible")
  c0bdfd72 ("sched/fair: Hide unused init_cfs_bandwidth() stub")
  378be384 ("sched: Add schedule_user() declaration")
  d55ebae3 ("sched: Hide unused sched_update_scaling()")

So in effect this is a revert of a revert and re-applies those
patches.

Fixes: 7aa55f2a

 ("sched/fair: Move unused stub functions to header")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>