ice: fix double-free of tx_buf skb

If ice_tso() or ice_tx_csum() fail, the error path in
ice_xmit_frame_ring() frees the skb, but the 'first' tx_buf still points
to it and is marked as valid (ICE_TX_BUF_SKB).
'next_to_use' remains unchanged, so the potential problem will
likely fix itself when the next packet is transmitted and the tx_buf
gets overwritten. But if there is no next packet and the interface is
brought down instead, ice_clean_tx_ring() -> ice_unmap_and_free_tx_buf()
will find the tx_buf and free the skb for the second time.

The fix is to reset the tx_buf type to ICE_TX_BUF_EMPTY in the error
path, so that ice_unmap_and_free_tx_buf().
Move the initialization of 'first' up, to ensure it's already valid in
case we hit the linearization error path.

The bug was spotted by AI while I had it looking for something else.
It also proposed an initial version of the patch.

I reproduced the bug and tested the fix by adding code to inject
failures, on a build with KASAN.

I looked for similar bugs in related Intel drivers and did not find any.

Fixes: d76a60ba7afb ("ice: Add support for VLANs and offloads")
Assisted-by: Claude:claude-4.6-opus-high Cursor
Signed-off-by: Michal Schmidt <mschmidt@redhat.com>