pppoe: remove rwlock usage

Like ppp_generic.c, convert the PPPoE socket hash table to use RCU for
lookups and a spinlock for updates. This removes rwlock usage and allows
lockless readers on the fast path.

- Mark hash table and list pointers as __rcu.
- Use spin_lock() to protect writers.
- Readers use rcu_dereference() under rcu_read_lock(). All known callers
  of get_item() already hold the RCU read lock, so no additional locking
  is needed.
- get_item() now uses refcount_inc_not_zero() instead of sock_hold() to
  safely take a reference. This prevents crashes if a socket is already
  in the process of being freed (sk_refcnt == 0).
- Set SOCK_RCU_FREE to defer socket freeing until after an RCU grace
  period.
- Move skb_queue_purge() into sk_destruct callback to ensure purge
  happens after an RCU grace period.

Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250828012018.15922-1-dqfext@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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