tcp: fix tcp_grow_skb() vs tstamps

I forgot to call tcp_skb_collapse_tstamp() in the
case we consume the second skb in write queue.

Neal suggested to create a common helper used by tcp_mtu_probe()
and tcp_grow_skb().

Fixes: 8ee602c6 ("tcp: try to send bigger TSO packets")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Link: https://lore.kernel.org/r/20240425193450.411640-1-edumazet@google.com


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