tcp: Fix a data-race around sysctl_tcp_rfc1337.

tcp: Fix a data-race around sysctl_tcp_rfc1337.

BugLink: https://bugs.launchpad.net/bugs/1988479
[ Upstream commit 0b484c91911e758e53656d570de58c2ed81ec6f2 ]

While reading sysctl_tcp_rfc1337, it can be changed concurrently.
Thus, we need to add READ_ONCE() to its reader.

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>