net/9p/usbg: fix handling of the failed kzalloc() memory allocation

On the linux-next, next-20241108 vanilla kernel, the coccinelle tool gave the
following error report:

./net/9p/trans_usbg.c:912:5-11: ERROR: allocation function on line 911 returns
NULL not ERR_PTR on failure

kzalloc() failure is fixed to handle the NULL return case on the memory exhaustion.

Fixes: a3be076 ("net/9p/usbg: Add new usb gadget function transport")
Cc: Michael Grzeschik <m.grzeschik@pengutronix.de>
Cc: Eric Van Hensbergen <ericvh@kernel.org>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Cc: Dominique Martinet <asmadeus@codewreck.org>
Cc: Christian Schoenebeck <linux_oss@crudebyte.com>
Cc: v9fs@lists.linux.dev
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Mirsad Todorovac <mtodorovac69@gmail.com>
Message-ID: <20241109211840.721226-2-mtodorovac69@gmail.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

Author: Mirsad Todorovac

net/9p/trans_usbg.c

@@ -909,9 +909,9 @@ static struct usb_function_instance *usb9pfs_alloc_instance(void)
 	usb9pfs_opts->buflen = DEFAULT_BUFLEN;
 
 	dev = kzalloc(sizeof(*dev), GFP_KERNEL);
-	if (IS_ERR(dev)) {
+	if (!dev) {
 		kfree(usb9pfs_opts);
-		return ERR_CAST(dev);
+		return ERR_PTR(-ENOMEM);
 	}
 
 	usb9pfs_opts->dev = dev;