Fix cross-tenant zone hijack in floating IP PTR API

_set_floatingip_reverse()’s lookup of the shared reverse-DNS zone matched by name only, across all tenants, with no ownership check. An attacker could pre-create a zone with a colliding name under their own tenant, causing a victim’s later, fully-authorized PTR-set to be silently written into the attacker’s zone. Scope this lookup (and its duplicate in _create_ptr_zone()’s race-condition fallback) to the managed-resource tenant; a name collision with another tenant’s zone now raises DuplicateZone instead of silently succeeding.