tcp: set and get per-socket timestamp

A timestamp can be set, only if a socket is in the repair mode.

This patch adds a new socket option TCP_TIMESTAMP, which allows to
get and set current tcp times stamp.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Cc: James Morris <jmorris@namei.org>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Cc: Patrick McHardy <kaber@trash.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
index e962faa..6b1ead0 100644
--- a/include/uapi/linux/tcp.h
+++ b/include/uapi/linux/tcp.h
@@ -111,6 +111,7 @@
 #define TCP_QUEUE_SEQ		21
 #define TCP_REPAIR_OPTIONS	22
 #define TCP_FASTOPEN		23	/* Enable FastOpen on listeners */
+#define TCP_TIMESTAMP		24
 
 struct tcp_repair_opt {
 	__u32	opt_code;