mds: do not scatter_writebehind in MIX state
mds: do not scatter_writebehind in MIX state

Replicas might come in while we're flushing and get a MIX state with
the old state.

Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@newdream.net>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph