Fixed the ceph get mdsmap assertion.

Fixed the ceph get mdsmap assertion.

    When we want to get mdsmap, we try to get_version()
    and the return value err = 0 means success.

    The assert verified r == 0. r would not change in this flow.
    It always meet assert and lead mon failure.

    I think this verify should be:
        assert(err == 0)
    It will help to check return value of get_version().

If you have any questions, feel free to let me know.
Thanks!

Signed-off-by: Vicente Cheng <freeze.bilsted@gmail.com>