[NET] random : secure_tcp_sequence_number should not assume CONFIG_KTIME_SCALAR

All 32 bits machines but i386 dont have CONFIG_KTIME_SCALAR. On these
machines, ktime.tv64 is more than 4 times the (correct) result given
by ktime_to_ns()

Again on these machines, using ktime_get_real().tv64 >> 6 give a
32bits rollover every 64 seconds, which is not wanted (less than the
120 s MSL)

Using ktime_to_ns() is the portable way to get nsecs from a ktime, and
have correct code.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>