ceph: direct requests in snapped namespace based on nonsnap parent

When making a request in the virtual snapdir or a snapped portion of the
namespace, we should choose the MDS based on the first nonsnap parent (and
its caps).  If that is not the best place, we will get forward hints to
find the right MDS in the cluster.  This fixes ESTALE errors when using
the .snap directory and namespace with multiple MDSs.

Signed-off-by: Sage Weil <sage@newdream.net>