net: mvneta: use buf->type to determine whether to dma-unmap

Now that we use a different buffer type for TSO headers, we can use
buf->type to determine whether the original buffer was DMA-mapped or
not. The rules are:

	MVNETA_TYPE_XDP_TX - from a DMA pool, no unmap is required
	MVNETA_TYPE_XDP_NDO - dma_map_single()'d
	MVNETA_TYPE_SKB - normal skbuff, dma_map_single()'d
	MVNETA_TYPE_TSO - from the TSO buffer area

This means we only need to call dma_unmap_single() on the XDP_NDO and
SKB types of buffer, and we no longer need the private IS_TSO_HEADER()
which relies on the TSO region being contiguously allocated.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>