mds: open+pin stray dirfrags on startup

mds: open+pin stray dirfrags on startup

This ensures that the stray dirfrags are always open, which in turn ensures
that whenever we add straydn items the rstats/fragstats will get updated
properly.  This is a better solution than d3d767a.

Now we can assert the stray dirfrag is open in
get_or_create_stray_dentry() instead of calling get_or_open_dirfrag().

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