ext4: Use rcu_barrier() on module unload.

The ext4 module uses rcu_call() thus it should use rcu_barrier()on
module unload.

The kmem cache ext4_pspace_cachep is sometimes free'ed using
call_rcu() callbacks.  Thus, we must wait for completion of call_rcu()
before doing kmem_cache_destroy().
Signed-off-by: NJesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -2909,7 +2909,11 @@ int __init init_ext4_mballoc(void)
 
 void exit_ext4_mballoc(void)
 {
-	/* XXX: synchronize_rcu(); */
+	/* 
+	 * Wait for completion of call_rcu()'s on ext4_pspace_cachep
+	 * before destroying the slab cache.
+	 */
+	rcu_barrier();
 	kmem_cache_destroy(ext4_pspace_cachep);
 	kmem_cache_destroy(ext4_ac_cachep);
 	kmem_cache_destroy(ext4_free_ext_cachep);