MDS: multifs: enable two filesystems to point to same pools if one of them is marked damaged
The 'damaged' flag on a filesystem prevents any MDS from being assigned a rank in that filesystem. While a filesystem is damaged, we should allow another filesystem to be created that targets the same pools (or possibly only the same data pools?), so that that filesystem could be used for repair (http://tracker.ceph.com/issues/15068).
The typical workflow would be that we had an existing filesystem 'A' that used pools 'data' and 'metadata'. It got marked damaged, and the user decided to create a new filesystem 'B' that uses pools 'newmetadata' and 'data'. They would then use repair tools to regenerate metadata into 'newmetadata', while avoiding the need to do any copying of stuff in the data pool.
Once they were happy with the new situation, they could then remove the old damaged filesystem. If something went wrong with repair and their old filesystem was in a "better than nothing" state, they could also remove the new filesystem and start the old one up again.
Because the damaged flag will have this new purpose, we will also need a command for the user to explicity set it (currently they can unset it with 'repaired' but it's only set initially by request of an MDS).