tcp: move tp->tcp_usec_ts to tcp_sock_read_txrx group

tp->tcp_usec_ts is a read mostly field, used in rx and tx fast paths.

Fixes: d5fed5ad ("tcp: reorganize tcp_sock fast path variables")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Coco Li <lixiaoyan@google.com>
Cc: Wei Wang <weiwan@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>