usb: subtle increased memory usage in u_serial

OK, the USB gadget serial driver actually has a couple of problems.  On
gs_open(), it always allocates and queues an additional QUEUE_SIZE (16)
worth of requests, so with a loop like this:

    i=1 ; while echo $i > /dev/ttyGS0 ; do let i++ ; done

eventually we run into OOM (Out of Memory).

Technically, it is not a leak as everything gets freed up when the USB
connection is broken, but not on gs_close().

With a USB device/gadget controller driver that has limited resources
(e.g., Marvell has a this MAX_XDS_FOR_TR_CALLS of 64 for transmit and
receive), so even after 4

    stty -F /dev/ttyGS0

we cannot transmit anymore.  We can still receive (not necessarily
reliably) as now we have 16 * 4 = 64 descriptors/buffers ready, but the
device is otherwise not usable.

Signed-off-by: Jim Sung <jsung@syncadence.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>