rbd: remove experimental designation from kernel layering

Support for kernel layering hasn't been considered experimental for
a few years now.  All the issues that I'm aware of were shaken out in
2014 and early 2015.  Moreover, most of that code was rewritten with
the addition of support for fancy striping.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>