net: gro: clear skb_shinfo(skb)->hwtstamps in napi_reuse_skb()

Some network drivers assume this field is zero after napi_get_frags().

We must clear it in napi_reuse_skb() otherwise the following can happen:

1) A packet is received, and skb_shinfo(skb)->hwtstamps is populated
   because a bit in the receive descriptor announced hwtstamp
   availability for this packet.

2) Packet is given to gro layer via napi_gro_frags().

3) Packet is merged to a prior one held in GRO queues.

4) skb is saved after some cleanup in napi->skb via a call
   to napi_reuse_skb().

5) Next packet is received 10 seconds later, gets the recycled skb
   from napi_get_frags().

6) The receive descriptor does not announce hwtstamp availability.
   Driver does not clear shinfo->hwtstamps.

7) We have in shinfo->hwtstamps an old timestamp.

Fixes: ac45f602 ("net: infrastructure for hardware time stamping")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://patch.msgid.link/20251015063221.4171986-1-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>