tcp: Fix a data-race around sysctl_tcp_retrans_collapse.

[ Upstream commit 1a63cb91 ]

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

Fixes: 1da177e4 ("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>