This commit adds support for smp_mb__after_srcu_read_unlock(), which,
when combined with a prior srcu_read_unlock(), implies a full memory
barrier.  No ordering is guaranteed to accesses between the two, and
placing accesses between is bad practice in any case.

Tests may be found at https://github.com/paulmckrcu/litmus in files
matching manual/kernel/C-srcu-mb-*.litmus.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>