mds: do not bump scatter stat lock in predirty_journal_parents

mds: do not bump scatter stat lock in predirty_journal_parents

If we're in the MIX state, we clearly can't touch this without screwing up
the delicate scatter/gather behavior.  If we're in, say, LOCK, there is
still no reason to update it.  One frag at least is local and auth if we
are in this code, but there may be other frags on other nodes.  This would
just make them appear stale when they are not.

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