tcp: Fix data-races around sysctl_tcp_slow_start_after_idle.

stable inclusion
from stable-v4.19.254
commit 3b26e11b07a09b31247688bec61e2925d4a571b6
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5Q0SQ


CVE: NA

--------------------------------

[ Upstream commit 4845b5713ab18a1bb6e31d1fbb4d600240b8b691 ]

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

Fixes: 35089bb2 ("[TCP]: Add tcp_slow_start_after_idle sysctl.")
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>