cxgb4/chcr: update SGL DMA unmap for USO

The FW_ETH_TX_EO_WR used for sending UDP Segmentation Offload (USO)
requests expects the headers to be part of the descriptor and the
payload to be part of the SGL containing the DMA mapped addresses.
Hence, the DMA address in the first entry of the SGL can start after
the packet headers. Currently, unmap_sgl() tries to unmap from this
wrong offset, instead of the originally mapped DMA address.

So, use existing unmap_skb() instead, which takes originally saved DMA
addresses as input. Update all necessary Tx paths to save the original
DMA addresses, so that unmap_skb() can unmap them properly.

v2:
- No change.

Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>