USB: gadget: zero allocate endpoint 0 buffers

Under some conditions, USB gadget devices can show allocated buffer
contents to a host.  Fix this up by zero-allocating them so that any
extra data will all just be zeros.

Reported-by: Szymon Heidrich <szymon.heidrich@gmail.com>
Tested-by: Szymon Heidrich <szymon.heidrich@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>