mm/hugetlb: clean up ARCH_HAS_GIGANTIC_PAGE

This moves the #ifdef in C code to a Kconfig dependency.  Also we move
the gigantic_page_supported() function to be arch specific.

This allows architectures to conditionally enable runtime allocation of
gigantic huge page.  Architectures like ppc64 supports different
gigantic huge page size (16G and 1G) based on the translation mode
selected.  This provides an opportunity for ppc64 to enable runtime
allocation only w.r.t 1G hugepage.

No functional change in this patch.

Link: http://lkml.kernel.org/r/1494995292-4443-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Cc: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 1dbbe38..fe53a3a 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -22,7 +22,7 @@
 	def_bool y
 	depends on 64BIT
 	# Options that are inherently 64-bit kernel only:
-	select ARCH_HAS_GIGANTIC_PAGE
+	select ARCH_HAS_GIGANTIC_PAGE if (MEMORY_ISOLATION && COMPACTION) || CMA
 	select ARCH_SUPPORTS_INT128
 	select ARCH_USE_CMPXCHG_LOCKREF
 	select HAVE_ARCH_SOFT_DIRTY