mon/MDSMonitor: make legacy 'newfs' command idempotent

We need to return success if we get a dup command.  Simply check whether
the fs is already enabled with the same pools and name.

Fixes: #8857
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2113 from ceph/wip-8857

mon/MDSMonitor: make legacy 'newfs' command idempotent

Reviewed-by: John Spray <john.spray@redhat.com>