ipv4: add kernel parameter tcpmhash_entries

This patch also adds a reference to ip-sysctl.txt
where TCP metrics setup is described
Signed-off-by: NFabian Frederick <fabf@skynet.be>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/kernel-parameters.txt

@@ -3412,6 +3412,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			neutralize any effect of /proc/sys/kernel/sysrq.
 			Useful for debugging.
 
+	tcpmhash_entries= [KNL,NET]
+			Set the number of tcp_metrics_hash slots.
+			Default value is 8192 or 16384 depending on total
+			ram pages. This is used to specify the TCP metrics
+			cache size. See Documentation/networking/ip-sysctl.txt
+			"tcp_no_metrics_save" section for more details.
+
 	tdfx=		[HW,DRM]
 
 	test_suspend=	[SUSPEND][,N]