rbd: fix rw option skipping logic

rbd: fix rw option skipping logic

We slip in an extraneous comma if { "rw": "rw" } happens to be the
first map_options pair:

    # ./rbd map -o rw,share foo
    /dev/rbd0

    -> 127.0.0.1:6789 name=admin,key=client.admin,,share rbd foo -

The kernel mount options parser can handle it, but fix it nonetheless.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>