s390/mm: fix BUG_ON in crst_table_upgrade

commit 8ab867cb upstream.

A 31-bit compat process can force a BUG_ON in crst_table_upgrade
with specific, invalid mmap calls, e.g.

   mmap((void*) 0x7fff8000, 0x10000, 3, 32, -1, 0)

The arch_get_unmapped_area[_topdown] functions miss an if condition
in the decision to do a page table upgrade.

[ms: Backport to 4.12, minor context change]

Fixes: 9b11c791 ("s390/mm: simplify arch_get_unmapped_area[_topdown]")
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>