net: Fix data-races around sysctl_fb_tunnels_only_for_init_net.

net: Fix data-races around sysctl_fb_tunnels_only_for_init_net.

BugLink: https://bugs.launchpad.net/bugs/1991717
[ Upstream commit af67508ea6cbf0e4ea27f8120056fa2efce127dd ]

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

Fixes: 79134e6ce2c9 ("net: do not create fallback tunnels for non-default namespaces")
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>