net: add skb_mstamp infrastructure

ktime_get() is too expensive on some cases, and we'd like to get
usec resolution timestamps in TCP stack.

This patch adds a light weight facility using a combination of
local_clock() and jiffies samples.

Instead of :

        u64 t0, t1;

        t0 = ktime_get();
        // stuff
        t1 = ktime_get();
        delta_us = ktime_us_delta(t1, t0);

use :
        struct skb_mstamp t0, t1;

        skb_mstamp_get(&t0);
        // stuff
        skb_mstamp_get(&t1);
        delta_us = skb_mstamp_us_delta(&t1, &t0);

Note : local_clock() might have a (bounded) drift between cpus.

Do not use this infra in place of ktime_get() without understanding the
issues.

Signed-off-by: Eric Dumazet <[email protected]>
Cc: Stephen Hemminger <[email protected]>
Cc: Yuchung Cheng <[email protected]>
Cc: Neal Cardwell <[email protected]>
Cc: Larry Brakmo <[email protected]>
Cc: Julian Anastasov <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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