USB: gadgetfs: fix checks of wTotalLength in config descriptors

Andrey Konovalov's fuzz testing of gadgetfs showed that we should
improve the driver's checks for valid configuration descriptors passed
in by the user.  In particular, the driver needs to verify that the
wTotalLength value in the descriptor is not too short (smaller
than USB_DT_CONFIG_SIZE).  And the check for whether wTotalLength is
too large has to be changed, because the driver assumes there is
always enough room remaining in the buffer to hold a device descriptor
(at least USB_DT_DEVICE_SIZE bytes).

This patch adds the additional check and fixes the existing check.  It
may do a little more than strictly necessary, but one extra check
won't hurt.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: Andrey Konovalov <andreyknvl@google.com>
CC: <stable@vger.kernel.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>