aio: fix rcu sparse warnings introduced by ioctx table lookup patch

Sseveral sparse warnings were caused by missing rcu_dereference() annotations
for dereferencing mm->ioctx_table.  Thankfully, none of those were actual bugs
as the deref was protected by a spin lock in all instances.
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>

fs/aio.c

@@ -470,7 +470,7 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
 	struct aio_ring *ring;
 
 	spin_lock(&mm->ioctx_lock);
-	table = mm->ioctx_table;
+	table = rcu_dereference(mm->ioctx_table);
 
 	while (1) {
 		if (table)
@@ -498,7 +498,7 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
 		table->nr = new_nr;
 
 		spin_lock(&mm->ioctx_lock);
-		old = mm->ioctx_table;
+		old = rcu_dereference(mm->ioctx_table);
 
 		if (!old) {
 			rcu_assign_pointer(mm->ioctx_table, table);
@@ -622,7 +622,7 @@ static void kill_ioctx(struct mm_struct *mm, struct kioctx *ctx)
 		struct kioctx_table *table;
 
 		spin_lock(&mm->ioctx_lock);
-		table = mm->ioctx_table;
+		table = rcu_dereference(mm->ioctx_table);
 
 		WARN_ON(ctx != table->table[ctx->id]);
 		table->table[ctx->id] = NULL;