tools/memory-model: Add smp_mb__after_srcu_read_unlock()

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>

tools/memory-model/linux-kernel.cat

@@ -49,7 +49,8 @@ let mb = ([M] ; fencerel(Mb) ; [M]) |
  * also affected by the fence.
  *)
 	([M] ; po-unlock-lock-po ;
-		[After-unlock-lock] ; po ; [M])
+		[After-unlock-lock] ; po ; [M]) |
+	([M] ; po? ; [Srcu-unlock] ; fencerel(After-srcu-read-unlock) ; [M])
 let gp = po ; [Sync-rcu | Sync-srcu] ; po?
 let strong-fence = mb | gp