s390/mm: add missing virt_to_pfn() etc. helper functions

Add all the missing little helper functions like virt_to_pfn(),
phys_to_page(), etc.  While we had a couple of these helper functions
like e.g. page_to_phys() the other functions were missing, which is
quite annoying if one is looking for exactly such a function.

Therefore finally add all those little helper functions.

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