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

mm/madvise.c

···
 		 */
 		size = page_size(compound_head(page));
 
-		if (PageHWPoison(page)) {
-			put_page(page);
-			continue;
-		}
-
 		if (behavior == MADV_SOFT_OFFLINE) {
 			pr_info("Soft offlining pfn %#lx at process virtual address %#lx\n",
 				 pfn, start);

mm/memory-failure.c

···
 		unlock_page(page);
 		put_page(page);
 		pr_info("soft offline: %#lx page already poisoned\n", pfn);
-		return -EBUSY;
+		return 0;
 	}
 
 	if (!PageHuge(page))
···
 		pr_info("soft offline: %#lx page already poisoned\n", pfn);
 		if (flags & MF_COUNT_INCREASED)
 			put_page(page);
-		return -EBUSY;
+		return 0;
 	}
 
 	get_online_mems();