ipv6: Get rid of RTNL for SIOCADDRT and RTM_NEWROUTE.

ipv6: Get rid of RTNL for SIOCADDRT and RTM_NEWROUTE.

Now we are ready to remove RTNL from SIOCADDRT and RTM_NEWROUTE.

The remaining things to do are

  1. pass false to lwtunnel_valid_encap_type_attr()
  2. use rcu_dereference_rtnl() in fib6_check_nexthop()
  3. place rcu_read_lock() before ip6_route_info_create_nh().

Let's complete the RTNL-free conversion.

When each CPU-X adds 100000 routes on table-X in a batch
concurrently on c7a.metal-48xl EC2 instance with 192 CPUs,

without this series:

  $ sudo ./route_test.sh
  ...
  added 19200000 routes (100000 routes * 192 tables).
  time elapsed: 191577 milliseconds.

with this series:

  $ sudo ./route_test.sh
  ...
  added 19200000 routes (100000 routes * 192 tables).
  time elapsed: 62854 milliseconds.

I changed the number of routes in each table (1000 ~ 100000)
and consistently saw it finish 3x faster with this series.

Note that now every caller of lwtunnel_valid_encap_type() passes
false as the last argument, and this can be removed later.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250418000443.43734-16-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>