ceph: fix debugfs entry, simplify fsid checks

We may first learn our fsid from any of the mon, osd, or mds maps
(whichever the monitor sends first).  Consolidate checks in a single
helper.  Initialize the client debugfs entry then, since we need the
fsid (and global_id) for the directory name.

Also remove dead mount code.

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