net-timestamp: optimize sock_tx_timestamp default path

Few packets have timestamping enabled. Exit sock_tx_timestamp quickly
in this common case.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/sock.h
Simple merge

diff --cc net/socket.c
Simple merge