rbd: omit 'rw' option during map

The ro and rw options were added in linux 3.7. To be compatible with older
kernels, don't specify rw. The default will probably always be rw, so this
should not present any problems in the future.

Reported-by: nicolasc <nicolas.canceill@surfsara.nl>
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>