mm/hmm: fix hmm_range_fault()'s handling of swapped out pages

mm/hmm: fix hmm_range_fault()'s handling of swapped out pages

hmm_range_fault() may return NULL pages because some of the pfns are equal
to HMM_PFN_NONE. This happens randomly under memory pressure. The reason
is during the swapped out page pte path, hmm_vma_handle_pte() doesn't
update the fault variable from cpu_flags, so it failed to call
hmm_vam_do_fault() to swap the page in.

The fix is to call hmm_pte_need_fault() to update fault variable.

Fixes: 74eee180b935 ("mm/hmm/mirror: device page fault handler")
Link: https://lore.kernel.org/r/20190815205227.7949-1-Philip.Yang@amd.com
Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: "Jérôme Glisse" <jglisse@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>