mds: warn, don't crash, on rfiles/rsubdirs underflow
mds: warn, don't crash, on rfiles/rsubdirs underflow

This doesn't fix the bug, but lets the mds at least start up.

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph