KVM: s390: return -EFAULT if copy_from_user() fails

When copy_from_user() fails, this code returns the number of bytes
remaining instead of a negative error code.  The positive number is
returned to the user but otherwise it is harmless.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>