rcu: Apply smp_mb__after_unlock_lock() to preserve grace periods

RCU must ensure that there is the equivalent of a full memory barrier
between any memory access preceding grace period and any memory access
following that same grace period, regardless of which CPU(s) happen to
execute the two memory accesses.  Therefore, downgrading UNLOCK+LOCK to
no longer imply a full memory barrier requires some adjustments to RCU.

This commit therefore adds smp_mb__after_unlock_lock() invocations as
needed after the RCU lock acquisitions that need to be part of a
full-memory-barrier UNLOCK+LOCK.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>