ipv6: use READ_ONCE()/WRITE_ONCE() on fib6_table->fib_seq

ipv6: use READ_ONCE()/WRITE_ONCE() on fib6_table->fib_seq

Using RTNL to protect ops->fib_rules_seq reads seems a big hammer.

Writes are protected by RTNL.
We can use READ_ONCE() when reading it.

Constify 'struct net' argument of fib6_tables_seq_read() and
fib6_rules_seq_read().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20241009184405.3752829-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>