page migration: Only migrate pages if allocation in the highest zone is possible

Address spaces contain an allocation flag that specifies restriction on the
zone for pages placed in the mapping.  I.e.  some device may require pages
to be allocated from a DMA zone.  Block devices may not be able to use
pages from HIGHMEM.

Memory policies and the common use of page migration works only on the
highest zone.  If the address space does not allow allocation from the
highest zone then the pages in the address space are not migratable simply
because we can only allocate memory for a specified node if we allow
allocation for the highest zone on each node.

Acked-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>