s390/mm: optimize copy_page

Always use the mvcl instruction to copy a page instead of mvpg or a
couple of mvc instructions.
Copying a huge page is 25% faster this way. Also bypass caches when
copying pages since only parts of a page will be used afterwards.
Especially when copying a huge page this would kick everything out
of the L1 and L2 data caches on a zEC12 machine.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>