ceph: remove outdated frag information

If directory fragments change, fill_inode() inserts new frags into
the fragtree, but it does not remove outdated frags from the fragtree.
This patch fixes it.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Reviewed-by: Sage Weil <sage@inktank.com>