tcp: move tcp_ns_to_ts() to net/ipv4/syncookies.c

tcp_ns_to_ts() is only used once from cookie_init_timestamp().

Also add the 'bool usec_ts' parameter to enable usec TS later.

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