mm: don't clear ->mapping in hmm_devmem_free

->mapping isn't even used by HMM users, and the field at the same offset
in the zone_device part of the union is declared as pad.  (Which btw is
rather confusing, as DAX uses ->pgmap and ->mapping from two different
sides of the union, but DAX doesn't use hmm_devmem_free).

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

mm/hmm.c

@@ -1383,8 +1383,6 @@ static void hmm_devmem_free(struct page *page, void *data)
 {
 	struct hmm_devmem *devmem = data;
 
-	page->mapping = NULL;
-
 	devmem->ops->free(devmem, page);
 }