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/20231218135837.3310403-4-willy@infradead.org
Fixes: 6100e34 ("mm, memory_failure: Teach memory_failure() about dev_pagemap pages")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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);
 	}