s390/mm: allow order 10 allocations

Get rid of FORCE_MAX_ZONEORDER which limited allocations to order 8 (= 1MB)
and use the default, which allows for order 10 (= 4MB) allocations.

Given that s390 allows less than the default this caused some memory
allocation problems more or less unique to s390 from time to time.

Note: this was originally introduced with commit 684de39b ("[S390]
Fix IPL from NSS.") in order to support Named Saved Segments, which
could start/end at an arbitrary 1 megabyte boundary and also before
support for sparsemem vmemmmap was enabled.

Since NSS support is gone, but sparsemem vmemmap support is available
this limitation can go away.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>