mm/slub: Replace sort_r() with sort() for debugfs stack trace sorting

The comparison function used to sort stack trace locations in debugfs
never relied on the third argument. Therefore, sort_r() is unnecessary.
Switch to sort() with a two-argument comparison function to keep the
code simple and aligned with the intended usage.

Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>