[nrf fromtree] Bluetooth: ATT: Fix conn parameter to req cb in att_reset

The conn pointer is still valid / not reused at this time and can be
used further up the stack as an identifer. This simplifies the API of
ATT, and fixes callbacks in GATT that pass on this value directly since
their API does not allow conn to be NULL.

Fixes #41794

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>

Author: alwa-nordic

doc/releases/release-notes-3.0.rst

@@ -44,7 +44,13 @@ Changes in this release
 Changes in this release
 =======================
 
-Removed APIs in this release:
+* GATT callbacks ``bt_gatt_..._func_t`` would previously be called with argument
+  ``conn = NULL`` in the event of a disconnect. This was not documented as part
+  of the API. This behavior is changed so the ``conn`` argument is provided as
+  normal when a disconnect occurs.
+
+Removed APIs in this release
+============================
 
 * The following Kconfig options related to radio front-end modules (FEMs) were
   removed:

subsys/bluetooth/host/att.c

@@ -2622,8 +2622,6 @@ static void att_reset(struct bt_att *att)
 		net_buf_unref(buf);
 	}
 
-	att->conn = NULL;
-
 	/* Notify pending requests */
 	while (!sys_slist_is_empty(&att->reqs)) {
 		struct bt_att_req *req;
@@ -2632,13 +2630,17 @@ static void att_reset(struct bt_att *att)
 		node = sys_slist_get_not_empty(&att->reqs);
 		req = CONTAINER_OF(node, struct bt_att_req, node);
 		if (req->func) {
-			req->func(NULL, BT_ATT_ERR_UNLIKELY, NULL, 0,
+			req->func(att->conn, BT_ATT_ERR_UNLIKELY, NULL, 0,
 				  req->user_data);
 		}
 
 		bt_att_req_free(req);
 	}
 
+	/* FIXME: `att->conn` is not reference counted. Consider using `bt_conn_ref`
+	 * and `bt_conn_unref` to follow convention.
+	 */
+	att->conn = NULL;
 	k_mem_slab_free(&att_slab, (void **)&att);
 }