igmp: Fix data-races around sysctl_igmp_llm_reports.

While reading sysctl_igmp_llm_reports, 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.

  if (ipv4_is_local_multicast(pmc->multiaddr) &&
      !READ_ONCE(net->ipv4.sysctl_igmp_llm_reports))

Fixes: df2cf4a7 ("IGMP: Inhibit reports for local multicast groups")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>