utils: compact-radix-tree: fix accidental cache line bouncing
utils: compact-radix-tree: fix accidental cache line bouncing

Whenever a node_head_ptr is assigned to nil_root, the _backref inside it is
overwritten. But since nil_root is shared between shards, this causes severe
cache line bouncing. (It was observed to reduce the total write throughput
of Scylla by 90% on a large NUMA machine).

This backreference is never read anyway, so fix this bug by not writing it.

Fixes #9252

Closes #9246

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla