s390/cio: Ensure the copied buf is NUL terminated

In the Linux kernel, the following vulnerability has been resolved: s390/cio: Ensure the copied buf is NUL terminated Currently, we allocate a lbuf-sized kernel buffer and copy lbuf from userspace to that buffer. Later, we use scanf on this buffer but we don't ensure that the string is terminated inside the buffer, this can lead to OOB read when using scanf. Fix this issue by using memdup_user_nul instead.