ipv6: mcast: annotate igmp6 timer expiry race

ipv6: mcast: annotate igmp6 timer expiry race

/proc/net/igmp6 walks IPv6 multicast memberships under RCU and reads
mca_work.timer.expires to print the remaining multicast timer. The
delayed-work timer can be updated concurrently.

Annotate the intentional lockless procfs snapshot with READ_ONCE().

Signed-off-by: Yuyang Huang <sigefriedhyy@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260609081113.7613-3-sigefriedhyy@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>