inet: annotate data-races around ifa->ifa_tstamp and ifa->ifa_cstamp

ifa->ifa_tstamp can be read locklessly.

Add appropriate READ_ONCE()/WRITE_ONCE() annotations.

Do the same for ifa->ifa_cstamp to prepare upcoming changes.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>