mm: add simple folio migration debugfs stats

Any improvements on enhancing success rates for folio migrations
can help overall performance and reduce memory fragmentation. Since
these counters have been found very valuable, add a simple a debugfs
interface to enable A/B testing for future enhancements without having
to use eBPF for introspection.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
1 file changed