mm: hugetlb: allow hugepages_supported to be architecture specific

s390 has a constant hugepage size, by setting HPAGE_SHIFT we also change
e.g. the pageblock_order, which should be independent in respect to
hugepage support.

With this patch every architecture is free to define how to check
for hugepage support.
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>

include/linux/hugetlb.h

@@ -460,15 +460,14 @@ static inline spinlock_t *huge_pte_lockptr(struct hstate *h,
 	return &mm->page_table_lock;
 }
 
-static inline bool hugepages_supported(void)
-{
-	/*
+#ifndef hugepages_supported
+/*
  * Some platform decide whether they support huge pages at boot
-	 * time. On these, such as powerpc, HPAGE_SHIFT is set to 0 when
-	 * there is no such support
+ * time. Some of them, such as powerpc, set HPAGE_SHIFT to 0
+ * when there is no such support
  */
-	return HPAGE_SHIFT != 0;
-}
+#define hugepages_supported() (HPAGE_SHIFT != 0)
+#endif
 
 #else	/* CONFIG_HUGETLB_PAGE */
 struct hstate {};