usb: usbtest: avoid integer overflow in alloc_sglist()

A large `nents' from userspace could overflow the allocation size,
leading to memory corruption.

| alloc_sglist()
| usbtest_ioctl()

Use kmalloc_array() to avoid the overflow.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>