ipv6: mcast: annotate data-races around mc->mca_sfcount[MCAST_EXCLUDE]

mc->mca_sfcount[MCAST_EXCLUDE] is read locklessly from
ipv6_chk_mcast_addr().

Add READ_ONCE() and WRITE_ONCE() annotations accordingly.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20241210183352.86530-3-edumazet@google.com


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