usb: usbtest: fix missing kfree(dev->buf) in usbtest_disconnect

      BUG: memory leak
unreferenced object 0xffff888055046e00 (size 256):
  comm "kworker/2:9", pid 2570, jiffies 4294942129 (age 1095.500s)
  hex dump (first 32 bytes):
    00 70 04 55 80 88 ff ff 18 bb 5a 81 ff ff ff ff  .p.U......Z.....
    f5 96 78 81 ff ff ff ff 37 de 8e 81 ff ff ff ff  ..x.....7.......
  backtrace:
    [<00000000d121dccf>] kmemleak_alloc_recursive
include/linux/kmemleak.h:43 [inline]
    [<00000000d121dccf>] slab_post_alloc_hook mm/slab.h:586 [inline]
    [<00000000d121dccf>] slab_alloc_node mm/slub.c:2786 [inline]
    [<00000000d121dccf>] slab_alloc mm/slub.c:2794 [inline]
    [<00000000d121dccf>] kmem_cache_alloc_trace+0x15e/0x2d0 mm/slub.c:2811
    [<000000005c3c3381>] kmalloc include/linux/slab.h:555 [inline]
    [<000000005c3c3381>] usbtest_probe+0x286/0x19d0
drivers/usb/misc/usbtest.c:2790
    [<000000001cec6910>] usb_probe_interface+0x2bd/0x870
drivers/usb/core/driver.c:361
    [<000000007806c118>]