mds: match bottom half of assilate_dirty_rstat_inodes with a dir flag

mds: match bottom half of assilate_dirty_rstat_inodes with a dir flag

We only do the assimilate_dirty_rstat_inodes if we do an update AND the
frag rstat was non-stale, but the bottom half (_finish) doesn't have the
same info to know whether we did it because the top half updates the
fragstat version.  Use a flag to indicate we've updated the dirfrag so
the bottom half will only run when needed.

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