Documentation: ip-sysctl.txt: Document tcp_fwmark_accept

This patch documents the tcp_fwmark_accept sysctl that was
added in 3.15.

Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -316,6 +316,17 @@ tcp_frto - INTEGER
 
 	By default it's enabled with a non-zero value. 0 disables F-RTO.
 
+tcp_fwmark_accept - BOOLEAN
+	If set, incoming connections to listening sockets that do not have a
+	socket mark will set the mark of the accepting socket to the fwmark of
+	the incoming SYN packet. This will cause all packets on that connection
+	(starting from the first SYNACK) to be sent with that fwmark. The
+	listening socket's mark is unchanged. Listening sockets that already
+	have a fwmark set via setsockopt(SOL_SOCKET, SO_MARK, ...) are
+	unaffected.
+
+	Default: 0
+
 tcp_invalid_ratelimit - INTEGER
 	Limit the maximal rate for sending duplicate acknowledgments
 	in response to incoming TCP packets that are for an existing