This was an ill-conceived feature that has been removed from Ceph. Do
this gracefully:
- reject attempts to specify a preferred_osd via the ioctl
- stop exposing this information via virtual xattrs
- always fill in -1 for requests, in case we talk to an older server
- don't calculate preferred_osd placements/pgids
Reviewed-by: Alex Elder <elder@inktank.com>
Signed-off-by: Sage Weil <sage@inktank.com>