net: annotate data-race around sk->sk_txrehash

sk_getsockopt() runs locklessly. This means sk->sk_txrehash
can be read while other threads are changing its value.

Other locations were handled in commit cb6cd2ce
("tcp: Change SYN ACK retransmit behaviour to account for rehash")

Fixes: 26859240

 ("txhash: Add socket option to control TX hash rethink behavior")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Akhmat Karakotov <hmukos@yandex-team.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>