s390/uaccess: Implement __get_kernel_nofault()/__put_kernel_nofault() with mvc

Use the mvc instruction in order to implement __get_kernel_nofault() and
__put_kernel_nofault(). Both functions have a source and destination
address where the code is supposed to read from and write to. Use the mvc
instruction to copy from source to destination instead of lg/stg like
instructions. This generates slightly better code.

Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>