memory-failure: fetch compound_head after pgmap_pfn_valid()

memory_failure_dev_pagemap() at the moment assumes base pages (e.g.
dax_lock_page()).  For devmap with compound pages fetch the
compound_head in case a tail page memory failure is being handled.

Currently this is a nop, but in the advent of compound pages in
dev_pagemap it allows memory_failure_dev_pagemap() to keep working.

Without this fix memory-failure handling (i.e.  MCEs on pmem) with
device-dax configured namespaces will regress (and crash).

Link: https://lkml.kernel.org/r/[email protected]
Reported-by: Jane Chu <[email protected]>
Signed-off-by: Joao Martins <[email protected]>
Reviewed-by: Naoya Horiguchi <[email protected]>
Reviewed-by: Dan Williams <[email protected]>
Reviewed-by: Muchun Song <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/memory-failure.c

@@ -1595,6 +1595,12 @@ static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
 		goto out;
 	}
 
+	/*
+	 * Pages instantiated by device-dax (not filesystem-dax)
+	 * may be compound pages.
+	 */
+	page = compound_head(page);
+
 	/*
 	 * Prevent the inode from being freed while we are interrogating
 	 * the address_space, typically this would be handled by