[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>