rbd: define image request originator flag

Define a flag indicating whether an image request originated from
the Linux block layer (from blk_fetch_request()) or whether it was
initiated in order to satisfy an object request for a child image
of a layered rbd device.  For image requests initiated by objects of
child images we'll save a pointer to the object request rather than
the Linux block request.

For now, only block requests are used.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>