net: Factorise setup_net() and cleanup_net().

When we roll back the changes made by struct pernet_operations.init(),
we execute mostly identical sequences in three places.

  * setup_net()
  * cleanup_net()
  * free_exit_list()

The only difference between the first two is which list and RCU helpers
to use.

In setup_net(), an ops could fail on the way, so we need to perform a
reverse walk from its previous ops in pernet_list.  OTOH, in cleanup_net(),
we iterate the full list from tail to head.

The former passes the failed ops to list_for_each_entry_continue_reverse().
It's tricky, but we can reuse it for the latter if we pass list_entry() of
the head node.

Also, synchronize_rcu() and synchronize_rcu_expedited() can be easily
switched by an argument.

Let's factorise the rollback part in setup_net() and cleanup_net().

In the next patch, ops_undo_list() will be reused for free_exit_list(),
and then two arguments (ops_list and hold_rtnl) will differ.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20250411205258.63164-2-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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