ipv6: mcast: annotate data-races around mca_flags

ipv6: mcast: annotate data-races around mca_flags

/proc/net/igmp6 walks IPv6 multicast memberships under RCU and
prints mca_flags without holding idev->mc_lock. The multicast paths
update the field while holding idev->mc_lock.

Annotate this intentional lockless snapshot with READ_ONCE() and the
matching writers with WRITE_ONCE().

Signed-off-by: Yuyang Huang <sigefriedhyy@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260609081113.7613-2-sigefriedhyy@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>