s390/mm: optimize page table upgrade routine

There is a maximum of two new tables allocated on page table
upgrade. Because we know that a loop the current implementation
is based on could be unrolled with some improvements:

  * upgrade from 3 to 5 levels happens in one go - without an
    unnecessary re-take of page_table_lock in-between;

  * page tables initialization moved out of the atomic code;

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>