mm/hmm: fix memremap.h, move dev_page_fault_t callback to hmm

The kbuild robot reported the following on a development branch that used
memremap.h in a new path:

   In file included from arch/m68k/include/asm/pgtable_mm.h:148:0,
                     from arch/m68k/include/asm/pgtable.h:5,
                     from include/linux/memremap.h:7,
                     from drivers//dax/bus.c:3:
    arch/m68k/include/asm/motorola_pgtable.h: In function 'pgd_offset':
 >> arch/m68k/include/asm/motorola_pgtable.h:199:11: error: dereferencing pointer to incomplete type 'const struct mm_struct'
      return mm->pgd + pgd_index(address);
               ^~

The ->page_fault() callback is specific to HMM.  Move it to 'struct
hmm_devmem' where the unusual asm/pgtable.h dependency can be contained in
include/linux/hmm.h.  Longer term refactoring this dependency out of HMM
is recommended, but in the meantime memremap.h remains generic.

Link: http://lkml.kernel.org/r/154534090899.3120190.6652620807617715272.stgit@dwillia2-desk3.amr.corp.intel.com
Fixes: 5042db43cc26 ("mm/ZONE_DEVICE: new type of ZONE_DEVICE memory...")
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: "Jérôme Glisse" <jglisse@redhat.com>
Cc: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/hmm.c b/mm/hmm.c
index 7895877..a04e4b8 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -1087,10 +1087,10 @@ struct hmm_devmem *hmm_devmem_add(const struct hmm_devmem_ops *ops,
 	devmem->pfn_first = devmem->resource->start >> PAGE_SHIFT;
 	devmem->pfn_last = devmem->pfn_first +
 			   (resource_size(devmem->resource) >> PAGE_SHIFT);
+	devmem->page_fault = hmm_devmem_fault;
 
 	devmem->pagemap.type = MEMORY_DEVICE_PRIVATE;
 	devmem->pagemap.res = *devmem->resource;
-	devmem->pagemap.page_fault = hmm_devmem_fault;
 	devmem->pagemap.page_free = hmm_devmem_free;
 	devmem->pagemap.altmap_valid = false;
 	devmem->pagemap.ref = &devmem->ref;
@@ -1141,10 +1141,10 @@ struct hmm_devmem *hmm_devmem_add_resource(const struct hmm_devmem_ops *ops,
 	devmem->pfn_first = devmem->resource->start >> PAGE_SHIFT;
 	devmem->pfn_last = devmem->pfn_first +
 			   (resource_size(devmem->resource) >> PAGE_SHIFT);
+	devmem->page_fault = hmm_devmem_fault;
 
 	devmem->pagemap.type = MEMORY_DEVICE_PUBLIC;
 	devmem->pagemap.res = *devmem->resource;
-	devmem->pagemap.page_fault = hmm_devmem_fault;
 	devmem->pagemap.page_free = hmm_devmem_free;
 	devmem->pagemap.altmap_valid = false;
 	devmem->pagemap.ref = &devmem->ref;