mm/slub: sort debugfs output by frequency of stack traces

Sort the output of debugfs alloc_traces and free_traces by the frequency
of allocation/freeing stack traces. Most frequently used stack traces
will be printed first, e.g. for easier memory leak debugging.

Signed-off-by: Oliver Glitta <glittao@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-and-tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>