can: gs_usb: gs_usb_open/close(): fix memory leak

In the Linux kernel, the following vulnerability has been resolved:

can: gs_usb: gs_usb_open/close(): fix memory leak

The gs_usb driver appears to suffer from a malady common to many USB
CAN adapter drivers in that it performs usb_alloc_coherent() to
allocate a number of USB request blocks (URBs) for RX, and then later
relies on usb_kill_anchored_urbs() to free them, but this doesn't
actually free them. As a result, this may be leaking DMA memory that's
been used by the driver.

This commit is an adaptation of the techniques found in the esd_usb2
driver where a similar design pattern led to a memory leak. It
explicitly frees the RX URBs and their DMA memory via a call to
usb_free_coherent(). Since the RX URBs were allocated in the
gs_can_open(), we remove them in gs_can_close() rather than in the
disconnect function as was done in esd_usb2.

For more information, see the 928150fad41b ("can: esd_usb2: fix memory
leak").