mm/migrate: fix migrate_pgmap_owner w/o CONFIG_MMU_NOTIFIER

mm/migrate: fix migrate_pgmap_owner w/o CONFIG_MMU_NOTIFIER

On x86_64, when CONFIG_MMU_NOTIFIER is not set/enabled, there is a
compiler error:

   mm/migrate.c: In function 'migrate_vma_collect':
   mm/migrate.c:2481:7: error: 'struct mmu_notifier_range' has no member named 'migrate_pgmap_owner'
     range.migrate_pgmap_owner = migrate->pgmap_owner;
          ^

Fixes: 998427b3ad2c ("mm/notifier: add migration invalidation type")
Reported-by: Randy Dunlap <[email protected]>
Signed-off-by: Ralph Campbell <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Tested-by: Randy Dunlap <[email protected]>
Acked-by: Randy Dunlap <[email protected]>
Cc: Jerome Glisse <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: "Jason Gunthorpe" <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>