fragtree_t: fix get_leaves_under()

If fragtree is (*^1, 1*^1) and we want leaves under frag 000*.
get_leaves_under() return frag 0*, frag 10* and frag 11*. This is
obviously wrong.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: don't scatter gather update bare-bones dirfrags

bare-bones dirfrag has not yet been fetched from the object store,
so we can't add it to the journal.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>