tcp: use skb->len instead of skb->truesize in tcp_can_ingest()

Some applications are stuck to the 20th century and still use
small SO_RCVBUF values.

After the blamed commit, we can drop packets especially
when using LRO/hw-gro enabled NIC and small MSS (1500) values.

LRO/hw-gro NIC pack multiple segments into pages, allowing
tp->scaling_ratio to be set to a high value.

Whenever the receive queue gets full, we can receive a small packet
filling RWIN, but with a high skb->truesize, because most NIC use 4K page
plus sk_buff metadata even when receiving less than 1500 bytes of payload.

Even if we refine how tp->scaling_ratio is estimated,
we could have an issue at the start of the flow, because
the first round of packets (IW10) will be sent based on
the initial tp->scaling_ratio (1/2)

Relax tcp_can_ingest() to use skb->len instead of skb->truesize,
allowing the peer to use final RWIN, assuming a 'perfect'
scaling_ratio of 1.

Fixes: 1d2fbaad ("tcp: stronger sk_rcvbuf checks")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250927092827.2707901-1-edumazet@google.com


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