This was sending lots of metadata ops to MDSs to persuade
them to migrate some subtrees, but that was flaky. Use
the shiny new rank pinning functionality instead.
Signed-off-by: John Spray <john.spray@redhat.com>
Previously a later remote call could end up executing
before the remote python program in open_background
had actually got as far as opening the file.
Fixes: http://tracker.ceph.com/issues/18661
Signed-off-by: John Spray <john.spray@redhat.com>