When a page has the poison bit set replace the PTE with a poison entry.
This causes the right error handling to be done later when a process runs
into it.

v2: add a new flag to not do that (needed for the memory-failure handler
later) (Fengguang)
v3: remove unnecessary is_migration_entry() test (Fengguang, Minchan)
Reviewed-by: NMinchan Kim <minchan.kim@gmail.com>
Reviewed-by: NWu Fengguang <fengguang.wu@intel.com>
Signed-off-by: NAndi Kleen <ak@linux.intel.com>