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: a3be076d ("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>