rbd: get rid of RBD_OBJ_WRITE_{FLAT,GUARD}

In preparation for moving OSD request allocation and submission into
object request state machines, get rid of RBD_OBJ_WRITE_{FLAT,GUARD}.
We would need to start in a new state, whether the request is guarded
or not.  Unify them into RBD_OBJ_WRITE_OBJECT and pass guard info
through obj_req->flags.

While at it, make our ENOENT handling a little more precise: only hide
ENOENT when it is actually expected, that is on delete.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>