net: add missing data-race annotation for sk_ll_usec

In a prior commit I forgot that sk_getsockopt() reads
sk->sk_ll_usec without holding a lock.

Fixes: 0dbffbb5

 ("net: annotate data race around sk_ll_usec")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>