gve: fix zero-length skb frag with header-split

gve: fix zero-length skb frag with header-split

When header split is enabled and a header-only packet is
received such as a pure TCP ACK, GVE will indicate an
RX SKB with a zero-length fragment. If this SKB is then
hairpinned and sent back out, the GVE TX path will emit
a zero-length descriptor. Hardware considers this
an illegal descriptor and stops the queue, causing a
TX timeout and interface reset.

Fix it by not adding the zero-length skb frag.

Cc: stable@vger.kernel.org
Fixes: 5e37d8254e7f ("gve: Add header split data path")
Suggested-by: Praveen Kaligineedi <pkaligineedi@google.com>
Co-developed-by: Ziwei Xiao <ziweixiao@google.com>
Signed-off-by: Ziwei Xiao <ziweixiao@google.com>
Signed-off-by: Jordan Rhee <jordanrhee@google.com>
Signed-off-by: Harshitha Ramamurthy <hramamurthy@google.com>
Link: https://patch.msgid.link/20260807224315.234152-2-hramamurthy@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>