rbd: fetch object order before using it

rbd_dev_v2_header_onetime() fetches striping information, and
checks whether the image can be read by compariing the stripe unit
to the object size. It determines the object size by shifting
the object order, which is 0 at this point since it has not been
read yet. Move the call to get the image size and object order
before rbd_dev_v2_header_onetime() so it is set before use.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
rbd: send snapshot context with writes

Sending the right snapshot context with each write is required for
snapshots to work. Due to the ordering of calls, the snapshot context
is never set for any requests. This causes writes to the current
version of the image to be reflected in all snapshots, which are
supposed to be read-only.

This happens because rbd_osd_req_format_write() sets the snapshot
context based on obj_request->img_request. At this point, however,
obj_request->img_request has not been set yet, to the snapshot context
is set to NULL. Fix this by moving rbd_img_obj_request_add(), which
sets obj_request->img_request, before the osd request formatting
calls.

This resolves:
    http://tracker.ceph.com/issues/5465

Reported-by: Karol Jurak <karol.jurak@gmail.com>
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>