mm/memory-failure: cast index to loff_t before shifting it

On 32-bit systems, we'll lose the top bits of index because arithmetic
will be performed in unsigned long instead of unsigned long long.  This
affects files over 4GB in size.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 6100e34 ("mm, memory_failure: Teach memory_failure() about dev_pagemap pages")
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Matthew Wilcox (Oracle)

mm/memory-failure.c

@@ -1704,7 +1704,7 @@ static void unmap_and_kill(struct list_head *to_kill, unsigned long pfn,
 		 * mapping being torn down is communicated in siginfo, see
 		 * kill_proc()
 		 */
-		loff_t start = (index << PAGE_SHIFT) & ~(size - 1);
+		loff_t start = ((loff_t)index << PAGE_SHIFT) & ~(size - 1);
 
 		unmap_mapping_range(mapping, start, size, 0);
 	}