mm: hwpoison: support recovery from ksm_might_need_to_copy()

When the kernel copies a page from ksm_might_need_to_copy(), but runs into
an uncorrectable error, it will crash since poisoned page is consumed by
kernel, this is similar to the issue recently fixed by Copy-on-write
poison recovery.

When an error is detected during the page copy, return VM_FAULT_HWPOISON
in do_swap_page(), and install a hwpoison entry in unuse_pte() when
swapoff, which help us to avoid system crash.  Note, memory failure on a
KSM page will be skipped, but still call memory_failure_queue() to be
consistent with general memory failure process, and we could support KSM
page recovery in the feature.

[[email protected]: enhance unuse_pte(), fix issue found by lkp]
  Link: https://lkml.kernel.org/r/[email protected]
[[email protected]: update changelog, alter ksm_might_need_to_copy(), restore unlikely() in unuse_pte()]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Kefeng Wang <[email protected]>
Reviewed-by: Naoya Horiguchi <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Tony Luck <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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