MDS: be accurate about stats in check_rstats
We see occasional crashes in CDir::check_rstats function, on
assert(get_num_head_items() == (fnode.fragstat.nfiles + fnode.fragstat.nsubdirs));
There's a problem with that assert: get_num_head_items() is only looking at items in cache. That means that if we do have any real stat issues, we're missing them in the noise. Fix the checks to be accurate.