tcp: annotate data-race around tcp_md5sig_pool_populated

stable inclusion
from stable-v4.19.262
commit 5c4e1b8939195fe27b05d791577f92445b139a3e
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5ZXGL


CVE: NA

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

[ Upstream commit aacd467c ]

tcp_md5sig_pool_populated can be read while another thread
changes its value.

The race has no consequence because allocations
are protected with tcp_md5sig_mutex.

This patch adds READ_ONCE() and WRITE_ONCE() to document
the race and silence KCSAN.

Reported-by: Abhishek Shah <abhishek.shah@columbia.edu>
Signed-off-by: Eric Dumazet <edumazet@google.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>