rcu: Express Tiny RCU updates in terms of RCU rather than RCU-sched

This commit renames Tiny RCU functions so that the lowest level of
functionality is RCU (e.g., synchronize_rcu()) rather than RCU-sched
(e.g., synchronize_sched()).  This provides greater naming compatibility
with Tree RCU, which will in turn permit more LoC removal once
the RCU-sched and RCU-bh update-side API is removed.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Fix Tiny call_rcu()'s EXPORT_SYMBOL() in response to a bug
  report from kbuild test robot. ]