usb: gadget: f_midi: Annotate struct f_midi with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct f_midi.
Additionally, since the element count member must be set before accessing
the annotated flexible array member, move its initialization earlier.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: John Keeping <john@keeping.me.uk>
Cc: Peter Chen <peter.chen@nxp.com>
Cc: Hulk Robot <hulkci@huawei.com>
Cc: Allen Pais <allen.lkml@gmail.com>
Cc: Will McVicker <willmcvicker@google.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Zhang Qilong <zhangqilong3@huawei.com>
Cc: linux-usb@vger.kernel.org
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230915195938.never.611-kees@kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

Author: kees

drivers/usb/gadget/function/f_midi.c

@@ -99,7 +99,7 @@ struct f_midi {
 	unsigned int in_last_port;
 	unsigned char free_ref;
 
-	struct gmidi_in_port	in_ports_array[/* in_ports */];
+	struct gmidi_in_port	in_ports_array[] __counted_by(in_ports);
 };
 
 static inline struct f_midi *func_to_midi(struct usb_function *f)
@@ -1349,6 +1349,7 @@ static struct usb_function *f_midi_alloc(struct usb_function_instance *fi)
 		status = -ENOMEM;
 		goto setup_fail;
 	}
+	midi->in_ports = opts->in_ports;
 
 	for (i = 0; i < opts->in_ports; i++)
 		midi->in_ports_array[i].cable = i;
@@ -1359,7 +1360,6 @@ static struct usb_function *f_midi_alloc(struct usb_function_instance *fi)
 		status = -ENOMEM;
 		goto midi_free;
 	}
-	midi->in_ports = opts->in_ports;
 	midi->out_ports = opts->out_ports;
 	midi->index = opts->index;
 	midi->buflen = opts->buflen;