ipv6: annotate data-races around cnf.disable_ipv6

commit d289ab65 upstream.

disable_ipv6 is read locklessly, add appropriate READ_ONCE()
and WRITE_ONCE() annotations.

v2: do not preload net before rtnl_trylock() in
    addrconf_disable_ipv6() (Jiri)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 4db783d6 ("ipv6: prevent NULL dereference in ip6_output()")
Signed-off-by: Sasha Levin <sashal@kernel.org>
[Ashwin: Regenerated the Patch for v5.15]
Signed-off-by: Ashwin Dayanand Kamat <ashwin.kamat@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>