hugetlb: add allocate function for hugepage migration

We can't use existing hugepage allocation functions to allocate hugepage
for page migration, because page migration can happen asynchronously with
the running processes and page migration users should call the allocation
function with physical addresses (not virtual addresses) as arguments.

ChangeLog since v3:
- unify alloc_buddy_huge_page() and alloc_buddy_huge_page_node()

ChangeLog since v2:
- remove unnecessary get/put_mems_allowed() (thanks to David Rientjes)

ChangeLog since v1:
- add comment on top of alloc_huge_page_no_vma()

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Reviewed-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>