CVE-2026-74344: bpf: Clear rb node linkage when freeing bpf_rb_root

In the Linux kernel, the following vulnerability has been resolved:

bpf: Clear rb node linkage when freeing bpfrbroot

bpfrbrootfree() detaches the root by copying the current rbrootcached and then replacing the live root with RBROOTCACHED. It then walks the copied root and drops each object contained in the tree.

This leaves the rb node state intact while dropping the object. If the object is refcounted and survives the drop, its bpfrbnodekern still contains an owner pointer to the freed root and stale rb tree linkage. If a later bpfrbroot allocation reuses the same address, bpfrbtreeremove() can incorrectly pass the owner check and call rberasecached() on a node whose rb pointers belong to the old tree.

Mirror the list draining behavior by marking nodes as busy while the root is being detached, then clear the rb node and release the owner before dropping the containing object. This makes surviving nodes unowned and safe to reject from remove or accept for a later add.