ipv6: make addrconf_wq single threaded

Both addrconf_verify_work() and addrconf_dad_work() acquire rtnl,
there is no point trying to have one thread per cpu.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240201173031.3654257-1-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>