igmp: Fix data-races around sysctl_igmp_max_msf.

igmp: Fix data-races around sysctl_igmp_max_msf.

BugLink: https://bugs.launchpad.net/bugs/1988479
[ Upstream commit 6ae0f2e553737b8cce49a1372573c81130ffa80e ]

While reading sysctl_igmp_max_msf, it can be changed concurrently.
Thus, we need to add READ_ONCE() to its readers.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>