Reinstate the delete_inode methods. We now make a copy of the superblock

Reinstate the delete_inode methods.  We now make a copy of the superblock
operations table for the MDS superblock so we don't affect other users of
that client filesystem type.

I also removed a duplicate call to rpc_unregister_service() in mds_cleanup.
This is what was causing the "rpc unable to unregister 12" type messages at
cleanup.