ipv6: make mc_forwarding atomic

ipv6: make mc_forwarding atomic

BugLink: https://bugs.launchpad.net/bugs/1969107
[ Upstream commit 145c7a793838add5e004e7d49a67654dc7eba147 ]

This fixes minor data-races in ip6_mc_input() and
batadv_mcast_mla_rtr_flags_softif_get_ipv6()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 4790998fdd0d0c682512af9b4a875bc80da73836)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>