ppp: remove rwlock usage

ppp: remove rwlock usage

In struct channel, the upl lock is implemented using rwlock_t,
protecting access to pch->ppp and pch->bridge.

As previously discussed on the list, using rwlock in the network fast
path is not recommended.
This patch replaces the rwlock with a spinlock for writers, and uses RCU
for readers.

- pch->ppp and pch->bridge are now declared as __rcu pointers.
- Readers use rcu_dereference_bh() under rcu_read_lock_bh().
- Writers use spin_lock() to update.

Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Link: https://patch.msgid.link/20250822012548.6232-1-dqfext@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>