mm,hwpoison: return 0 if the page is already poisoned in soft-offline

Currently, there is an inconsistency when calling soft-offline from
different paths on a page that is already poisoned.

1) madvise:

        madvise_inject_error skips any poisoned page and continues
        the loop.
        If that was the only page to madvise, it returns 0.

2) /sys/devices/system/memory/:

        When calling soft_offline_page_store()->soft_offline_page(),
        we return -EBUSY in case the page is already poisoned.
        This is inconsistent with a) the above example and b)
        memory_failure, where we return 0 if the page was poisoned.

Fix this by dropping the PageHWPoison() check in madvise_inject_error, and
let soft_offline_page return 0 if it finds the page already poisoned.

Please, note that this represents a user-api change, since now the return
error when calling soft_offline_page_store()->soft_offline_page() will be
different.

Signed-off-by: Oscar Salvador <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Naoya Horiguchi <[email protected]>
Cc: "Aneesh Kumar K.V" <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Aristeu Rozanski <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Dmitry Yakunin <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Qian Cai <[email protected]>
Cc: Tony Luck <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

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