relocate() called forget(), dropping the node's by_uid mapping, so
the re-enumeration assigned a fresh inode. The kernel had already
carried the renamed dentry onto the old one and holds it for the
entry TTL, so lookup/getattr/opendir resolved to an inode entries
no longer held and returned ENOENT — while readdir walked children
and reported the new inode. A renamed directory listed in its
parent but could not be entered until the TTL expired.
Rewrite the node in place and invalidate both parents' listings
instead. Also stops the moved node's DB row being deleted and
recreated on every move, which the B1 data-loss fix worked around
nvrl