ipv4: Fix data-races around sysctl_fib_notify_on_flag_change.

ipv4: Fix data-races around sysctl_fib_notify_on_flag_change.

BugLink: https://bugs.launchpad.net/bugs/1989218
[ Upstream commit 96b9bd8c6d125490f9adfb57d387ef81a55a103e ]

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

Fixes: 680aea08e78c ("net: ipv4: Emit notification when fib hardware flags are changed")
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>