hugetlb: restrict hugepage_migration_support() to x86_64

hugetlb: restrict hugepage_migration_support() to x86_64

Currently hugepage migration is available for all archs which support
pmd-level hugepage, but testing is done only for x86_64 and there're
bugs for other archs.  So to avoid breaking such archs, this patch
limits the availability strictly to x86_64 until developers of other
archs get interested in enabling this feature.

Simply disabling hugepage migration on non-x86_64 archs is not enough to
fix the reported problem where sys_move_pages() hits the BUG_ON() in
follow_page(FOLL_GET), so let's fix this by checking if hugepage
migration is supported in vma_migratable().

Signed-off-by: Naoya Horiguchi <[email protected]>
Reported-by: Michael Ellerman <[email protected]>
Tested-by: Michael Ellerman <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Tony Luck <[email protected]>
Cc: Russell King <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: James Hogan <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: David Miller <[email protected]>
Cc: <[email protected]>	[3.12+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>