s390/mm: simplify arch_get_unmapped_area[_topdown]

With TASK_SIZE now reflecting the maximum size of the address space for
a process the code for arch_get_unmapped_area[_topdown] can be simplified.
Just let the logic pick a suitable address and deal with the page table
upgrade after the address has been selected.

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