sfc: Extend struct efx_tx_buffer to allow pushing option descriptors

The TX path firmware for EF10 supports 'option descriptors' to control
offloads and various other features.  Add a flag and field for these
in struct efx_tx_buffer, and don't treat them as DMA descriptors on
completion.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
diff --git a/drivers/net/ethernet/sfc/farch.c b/drivers/net/ethernet/sfc/farch.c
index 842f92e..65073aa 100644
--- a/drivers/net/ethernet/sfc/farch.c
+++ b/drivers/net/ethernet/sfc/farch.c
@@ -325,6 +325,8 @@
 		txd = efx_tx_desc(tx_queue, write_ptr);
 		++tx_queue->write_count;
 
+		EFX_BUG_ON_PARANOID(buffer->flags & EFX_TX_BUF_OPTION);
+
 		/* Create TX descriptor ring entry */
 		BUILD_BUG_ON(EFX_TX_BUF_CONT != 1);
 		EFX_POPULATE_QWORD_4(*txd,