ipv6: Remove rcu_read_lock() in fib6_get_table().

ipv6: Remove rcu_read_lock() in fib6_get_table().

Once allocated, the IPv6 routing table is not freed until
netns is dismantled.

fib6_get_table() uses rcu_read_lock() while iterating
net->ipv6.fib_table_hash[], but it's not needed and
rather confusing.

Because some callers have this pattern,

  table = fib6_get_table();

  rcu_read_lock();
  /* ... use table here ... */
  rcu_read_unlock();

  [ See: addrconf_get_prefix_route(), ip6_route_del(),
         rt6_get_route_info(), rt6_get_dflt_router() ]

and this looks illegal but is actually safe.

Let's remove rcu_read_lock() in fib6_get_table() and pass true
to the last argument of hlist_for_each_entry_rcu() to bypass
the RCU check.

Note that protection is not needed but RCU helper is used to
avoid data-race.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250516022759.44392-2-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>