crimson/os/seastore/object_data_handler: we don't have to maintain the

crimson/os/seastore/object_data_handler: we don't have to maintain the
symmetric indirect lba relationship at the time of clone

OP_CLONE is done in the following way:
1. First, swap the layout of the head onode and the clone onode, so that
   clone onode's object_data, omap_root, xattr_root and log_root all
   point to the head onode's corresponding fields;
2. Do SeaStore::_clone() from the clone onode to the head onode, which
   is exactly what rollback is done.

This makes the code of ObjectDataHandler::clone() and
ObjectDataHandler::copy_on_write() even simpler, and can facilitate the
clone/rollback scenarios when the "128-bit" lba key layout is involved.

Signed-off-by: Xuehan Xu <xuxuehan@qianxin.com>