[TCP]: secure_tcp_sequence_number() should not use a too fast clock

TCP V4 sequence numbers are 32bits, and RFC 793 assumed a 250 KHz clock.
In order to follow network speed increase, we can use a faster clock, but
we should limit this clock so that the delay between two rollovers is
greater than MSL (TCP Maximum Segment Lifetime : 2 minutes)

Choosing a 64 nsec clock should be OK, since the rollovers occur every
274 seconds.

Problem spotted by Denys Fedoryshchenko

[ This bug was introduced by f8595815 ]
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/char/random.c

@@ -1550,11 +1550,13 @@ __u32 secure_tcp_sequence_number(__be32 saddr, __be32 daddr,
 	 *	As close as possible to RFC 793, which
 	 *	suggests using a 250 kHz clock.
 	 *	Further reading shows this assumes 2 Mb/s networks.
-	 *	For 10 Gb/s Ethernet, a 1 GHz clock is appropriate.
-	 *	That's funny, Linux has one built in!  Use it!
-	 *	(Networks are faster now - should this be increased?)
+	 *	For 10 Mb/s Ethernet, a 1 MHz clock is appropriate.
+	 *	For 10 Gb/s Ethernet, a 1 GHz clock should be ok, but
+	 *	we also need to limit the resolution so that the u32 seq
+	 *	overlaps less than one time per MSL (2 minutes).
+	 *	Choosing a clock of 64 ns period is OK. (period of 274 s)
 	 */
-	seq += ktime_get_real().tv64;
+	seq += ktime_get_real().tv64 >> 6;
 #if 0
 	printk("init_seq(%lx, %lx, %d, %d) = %d\n",
 	       saddr, daddr, sport, dport, seq);