nouveau: fix migrate_to_ram() for faulting page

Commit 16ce101 ("mm/memory.c: fix race when faulting a device private
page") changed the migrate_to_ram() callback to take a reference on the
device page to ensure it can't be freed while handling the fault. 
Unfortunately the corresponding update to Nouveau to accommodate this
change was inadvertently dropped from that patch causing GPU to CPU
migration to fail so add it here.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 16ce101 ("mm/memory.c: fix race when faulting a device private page")
Signed-off-by: Alistair Popple <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Ralph Campbell <[email protected]>
Cc: Lyude Paul <[email protected]>
Cc: Ben Skeggs <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: apopple-nvidia

drivers/gpu/drm/nouveau/nouveau_dmem.c

@@ -176,6 +176,7 @@ static vm_fault_t nouveau_dmem_migrate_to_ram(struct vm_fault *vmf)
 		.src		= &src,
 		.dst		= &dst,
 		.pgmap_owner	= drm->dev,
+		.fault_page	= vmf->page,
 		.flags		= MIGRATE_VMA_SELECT_DEVICE_PRIVATE,
 	};