mds: simplify journaled subtree_map
mds: simplify journaled subtree_map

We may have subtrees split locally due to migrations that are just getting
started or stopped.  Simplify the map we journal to disk.  Among other
things this makes the replay check simple (it can compare against the
current "live" map).

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

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