hw/net: fix e1000e/igb ip_len inflation by Ethernet minimum-frame padding

When a guest transmits a short Ethernet frame, iov_size() returns the padded wire length including any bytes added to reach the Ethernet minimum frame size of 60 bytes. net_tx_pkt_rebuild_payload() uses this inflated size as payload_len. net_tx_pkt_update_ip_hdr_checksum() then overwrites the IPv4 Total Length field with payload_len + l3_hdr_len, inflating it by the padding. The receiver interprets Ethernet padding as IP payload, producing a malformed packet. Fix by removing the ip_len write from net_tx_pkt_update_ip_hdr_checksum() so it only recomputes the checksum, and moving the ip_len assignment into net_tx_pkt_update_ip_checksums() where it is only performed for TSO (where ip_len must be derived from payload_len since the guest sets ip_len=0 per Intel 82574 datasheet §7.3.4 for super-packets the host will segment). Both e1000e and igb already call net_tx_pkt_update_ip_hdr_checksum() from their IXSM paths, so both are corrected by this single common- layer change. Signed-off-by: Sanjeeva Yerrapureddy <y.sanjeevreddy@gmail.com> Reivewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp> Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com> Message-ID: <20260629-net-tx-pkt-ip-length-padding-v5-1-16760e30252e@gmail.com> Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>