net: Fix a data-race around sysctl_tstamp_allow_data.

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

Fixes: b245be1f

 ("net-timestamp: no-payload only sysctl")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>