mds: make get_dirfrags_under behave when dirfragtree is not coherent with dirfrag set
mds: make get_dirfrags_under behave when dirfragtree is not coherent with dirfrag set

This is (currently) the case during replay/resolve, although it's not
clear that it should be.

Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@newdream.net>

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