mds rejoin: invented dirfrags (MDCache.cc:3469)
invented dirfrags have version=0; subsequent modification of dentries/inodes predirty a bad (small) version #.
mds: open undef dirfrags during rejoin
Any invented dirfrags have a version of 0. This will cause problems later
if we pre_dirty() anything in that dir because the dir version won't be
in sync (it'll be way too small). Also, we can do that at any point,
e.g. when flushing dirty caps, and aren't allowed to delay, so we need to
load those dirfrags now.
In theory we could read only the fnode and not all the dentries, but we
may as well. We should be more careful about memory that this patch is,
Signed-off-by: Sage Weil <firstname.lastname@example.org>