tcp: Fix data-races around sysctl_tcp_base_mss.

stable inclusion
from stable-v5.10.134
commit 514d2254c7b8aa2d257f5ffc79f0d96be2d6bfda
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5ZVR7

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=514d2254c7b8aa2d257f5ffc79f0d96be2d6bfda



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[ Upstream commit 88d78bc0 ]

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

Fixes: 5d424d5a ("[TCP]: MTU probing")
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: Zheng Zengkai <zhengzengkai@huawei.com>
Reviewed-by: Wei Li <liwei391@huawei.com>