revert "s390/mm: make hugepages_supported a boot time decision"

Heiko noticed that the current check for hugepage support on s390 is a
little bit too harsh as systems which do not support will crash.

The reason is that pageblock_order can now get negative when we set
HPAGE_SHIFT to 0.  To avoid all this and to avoid opening another can of
worms with enabling HUGETLB_PAGE_SIZE_VARIABLE I think it would be best
to simply allow architectures to define their own hugepages_supported().

Revert bea41197 ("s390/mm: make hugepages_supported a boot time
decision") in preparation.

Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>