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>