mds: preserve stale state on import; some cleanup

mds: preserve stale state on import; some cleanup

Our new invariant is that MIX_STALE always implies is_stale().  And on
import, if is_stale(), MIX becomes MIX_STALE.  This ensures that a replica
that we put into MIX_STALE doesn't turn back into MIX if we import it
and take the auth's state in CInode::decode_import().

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