rbd: fix read-only option name

The name of the "read-only" mapping option was inadvertently changed
in this commit:

    f84344f3

 rbd: separate mapping info in rbd_dev

Revert that hunk to return it to what it should be.

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