mm, hugetlb: schedule when potentially allocating many hugepages

mm, hugetlb: schedule when potentially allocating many hugepages

A few hugetlb allocators loop while calling the page allocator and can
potentially prevent rescheduling if the page allocator slowpath is not
utilized.

Conditionally schedule when large numbers of hugepages can be allocated.

Anshuman:
 "Fixes a task which was getting hung while writing like 10000 hugepages
  (16MB on POWER8) into /proc/sys/vm/nr_hugepages."

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: David Rientjes <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Tested-by: Anshuman Khandual <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: "Aneesh Kumar K.V" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>