net: ensure correct skb->tstamp in various fragmenters

Thomas found that some forwarded packets would be stuck
in FQ packet scheduler because their skb->tstamp contained
timestamps far in the future.

We thought we addressed this point in commit 8203e2d8
("net: clear skb->tstamp in forwarding paths") but there
is still an issue when/if a packet needs to be fragmented.

In order to meet EDT requirements, we have to make sure all
fragments get the original skb->tstamp.

Note that this original skb->tstamp should be zero in
forwarding path, but might have a non zero value in
output path if user decided so.

Fixes: fb420d5d

 ("tcp/fq: move back to CLOCK_MONOTONIC")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Thomas Bartschies <Thomas.Bartschies@cvk.de>
Signed-off-by: David S. Miller <davem@davemloft.net>