rbd: drop dev reference on error in rbd_open()

If a read-only rbd device is opened for writing in rbd_open(), it
returns without dropping the just-acquired device reference.

Fix this by moving the read-only check before getting the reference.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
(cherry picked from commit 340c7a2b2c9a2da640af28a8c196356484ac8b50)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>