s390/setup: correct 4-level kernel page table detection

Fix calculation to decide if a 4-level kernel page table is required.
Git commit c972cc60 "s390/vmalloc: have separate modules area"
added the separate module area which reduces the size of the vmalloc
area but fails to take it into account for the 3 vs 4 level page table
decision.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>