ipv4: igmp: annotate data-races around in_dev->mc_count

ipv4: igmp: annotate data-races around in_dev->mc_count

/proc/net/igmp walks the multicast list for IPv4 interfaces locklessly
under RCU and prints state->in_dev->mc_count. Concurrently, device
init/destruction and multicast join/leave paths update the count
under the RTNL lock. Fix this intentional lockless snapshot by
annotating the read with READ_ONCE() and the updates with WRITE_ONCE().

Signed-off-by: Yuyang Huang <yuyanghuang@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260605014318.3890804-2-yuyanghuang@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>