ceph: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
[idryomov@gmail.com: amended "Older OSDs" comment]
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>