Kernel warnings via contended RCU bypass flush

Concurrency flaw in Linux allows local users to trigger kernel warnings through cache dropping when two RCU build options are enabled. lazy_rcu_shrink_scan incorrectly invokes the blocking rcu_nocb_flush_bypass path for another CPU, and a failed bypass-lock attempt leaves contention accounting that trips checks in rcu_nocb_bypass_lock and rcu_nocb_wait_contended. Reachability requires CONFIG_RCU_NOCB_CPU_DEFAULT_ALL and CONFIG_RCU_LAZY, plus administrative access to request cache dropping; the published consequence is warning generation rather than a documented crash.