net: Fix data-races around sysctl_optmem_max.

net: Fix data-races around sysctl_optmem_max.

BugLink: https://bugs.launchpad.net/bugs/1991717
[ Upstream commit 7de6d09f51917c829af2b835aba8bb5040f8e86a ]

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

Fixes: 1da177e4c3f4 ("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>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>