cxgb3: Fix length calculation in write_ofld_wr() on 32-bit architectures

The length calculation here is now invalid on 32-bit architectures,
since sk_buff::tail is a pointer and sk_buff::transport_header is
an integer offset:

drivers/net/ethernet/chelsio/cxgb3/sge.c: In function 'write_ofld_wr':
drivers/net/ethernet/chelsio/cxgb3/sge.c:1603:9: warning: passing argument 4 of 'make_sgl' makes integer from pointer without a cast [enabled by default]
         adap->pdev);
         ^
drivers/net/ethernet/chelsio/cxgb3/sge.c:964:28: note: expected 'unsigned int' but argument is of type 'sk_buff_data_t'
 static inline unsigned int make_sgl(const struct sk_buff *skb,
                            ^

Use the appropriate skb accessor functions.

Compile-tested only.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Fixes: 1a37e412 ('net: Use 16bits for *_headers fields of struct skbuff')
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/chelsio/cxgb3/sge.c

@@ -1599,7 +1599,8 @@ static void write_ofld_wr(struct adapter *adap, struct sk_buff *skb,
 	flits = skb_transport_offset(skb) / 8;
 	sgp = ndesc == 1 ? (struct sg_ent *)&d->flit[flits] : sgl;
 	sgl_flits = make_sgl(skb, sgp, skb_transport_header(skb),
-			     skb->tail - skb->transport_header,
+			     skb_tail_pointer(skb) -
+			     skb_transport_header(skb),
 			     adap->pdev);
 	if (need_skb_unmap()) {
 		setup_deferred_unmapping(skb, adap->pdev, sgp, sgl_flits);