tcp: more struct tcp_sock adjustments

tp->recvmsg_inq is used from tcp recvmsg() thus should
be in tcp_sock_read_rx group.

tp->tcp_clock_cache and tp->tcp_mstamp are written
both in rx and tx paths, thus are better placed
in tcp_sock_write_txrx group.

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