apparmor: aa_buffer: Convert 1-element array to flexible array

In the ongoing effort to convert all fake flexible arrays to proper
flexible arrays, replace aa_buffer's 1-element "buffer" member with a
flexible array.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/lsm.c

@@ -46,7 +46,7 @@ int apparmor_initialized;
 
 union aa_buffer {
 	struct list_head list;
-	char buffer[1];
+	DECLARE_FLEX_ARRAY(char, buffer);
 };
 
 #define RESERVE_COUNT 2
@@ -1649,7 +1649,7 @@ char *aa_get_buffer(bool in_atomic)
 		list_del(&aa_buf->list);
 		buffer_count--;
 		spin_unlock(&aa_buffers_lock);
-		return &aa_buf->buffer[0];
+		return aa_buf->buffer;
 	}
 	if (in_atomic) {
 		/*
@@ -1672,7 +1672,7 @@ char *aa_get_buffer(bool in_atomic)
 		pr_warn_once("AppArmor: Failed to allocate a memory buffer.\n");
 		return NULL;
 	}
-	return &aa_buf->buffer[0];
+	return aa_buf->buffer;
 }
 
 void aa_put_buffer(char *buf)
@@ -1749,7 +1749,7 @@ static int __init alloc_buffers(void)
 			destroy_buffers();
 			return -ENOMEM;
 		}
-		aa_put_buffer(&aa_buf->buffer[0]);
+		aa_put_buffer(aa_buf->buffer);
 	}
 	return 0;
 }