igmp: Fix data-races around sysctl_igmp_qrv.

In the Linux kernel, the following vulnerability has been resolved:

igmp: Fix data-races around sysctl_igmp_qrv.

While reading sysctl_igmp_qrv, it can be changed concurrently.

Thus, we need to add READ_ONCE() to its readers.

This test can be packed into a helper, so such changes will be in the

follow-up series after net is merged into net-next.

qrv ?: READ_ONCE(net->ipv4.sysctl_igmp_qrv);