tcp: stronger sk_rcvbuf checks

tcp: stronger sk_rcvbuf checks

Currently, TCP stack accepts incoming packet if sizes of receive queues
are below sk->sk_rcvbuf limit.

This can cause memory overshoot if the packet is big, like an 1/2 MB
BIG TCP one.

Refine the check to take into account the incoming skb truesize.

Note that we still accept the packet if the receive queue is empty,
to not completely freeze TCP flows in pathological conditions.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250711114006.480026-8-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>