ipv6: sr: add documentation file for per-interface sysctls

This patch adds documentation for some SR-related per-interface
sysctls.
Signed-off-by: NDavid Lebrun <david.lebrun@uclouvain.be>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/seg6-sysctl.txt

+/proc/sys/net/conf/<iface>/seg6_* variables:
+
+seg6_enabled - BOOL
+	Accept or drop SR-enabled IPv6 packets on this interface.
+
+	Relevant packets are those with SRH present and DA = local.
+
+	0 - disabled (default)
+	not 0 - enabled
+
+seg6_require_hmac - INTEGER
+	Define HMAC policy for ingress SR-enabled packets on this interface.
+
+	-1 - Ignore HMAC field
+	0 - Accept SR packets without HMAC, validate SR packets with HMAC
+	1 - Drop SR packets without HMAC, validate SR packets with HMAC
+
+	Default is 0.