s390/uaccess: Use asm goto for __mvc_kernel_nofault()

Use asm goto for __mvc_kernel_nofault() if available. This generates
slightly better code, since the error checking happens implicitly with
the goto (aka exception) and the good path comes without any checks and
branches.

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>