tcp: Fix a data-race around sysctl_tcp_probe_interval.

stable inclusion
from stable-v4.19.254
commit b3798d3519eda9c409bb0815b0102f27ec42468d
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5Q0SQ


CVE: NA

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[ Upstream commit 2a85388f ]

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

Fixes: 05cbc0db ("ipv4: Create probe timer for tcp PMTU as per RFC4821")
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: Yongqiang Liu <liuyongqiang13@huawei.com>