mm/page_alloc: add alloc_contig_pages()

HugeTLB helper alloc_gigantic_page() implements fairly generic
allocation method where it scans over various zones looking for a large
contiguous pfn range before trying to allocate it with
alloc_contig_range().

Other than deriving the requested order from 'struct hstate', there is
nothing HugeTLB specific in there.  This can be made available for
general use to allocate contiguous memory which could not have been
allocated through the buddy allocator.

alloc_gigantic_page() has been split carving out actual allocation
method which is then made available via new alloc_contig_pages() helper
wrapped under CONFIG_CONTIG_ALLOC.  All references to 'gigantic' have
been replaced with more generic term 'contig'.  Allocated pages here
should be freed with free_contig_range() or by calling __free_page() on
each allocated page.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Anshuman Khandual <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Pavel Tatashin <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: David Hildenbrand <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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