s390/qeth: fix overestimated count of buffer elements

qeth_get_elements_for_range() doesn't know how to handle a 0-length
range (ie. start == end), and returns 1 when it should return 0.
Such ranges occur on TSO skbs, where the L2/L3/L4 headers (and thus all
of the skb's linear data) are skipped when mapping the skb into regular
buffer elements.

This overestimation may cause several performance-related issues:
1. sub-optimal IO buffer selection, where the next buffer gets selected
   even though the skb would actually still fit into the current buffer.
2. forced linearization, if the element count for a non-linear skb
   exceeds QETH_MAX_BUFFER_ELEMENTS.

Rather than modifying qeth_get_elements_for_range() and adding overhead
to every caller, fix up those callers that are in risk of passing a
0-length range.

Fixes: 2863c613 ("qeth: refactor calculation of SBALE count")
Signed-off-by: NJulian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/s390/net/qeth_core_main.c

@@ -3898,10 +3898,12 @@ EXPORT_SYMBOL_GPL(qeth_get_elements_for_frags);
 int qeth_get_elements_no(struct qeth_card *card,
 		     struct sk_buff *skb, int extra_elems, int data_offset)
 {
-	int elements = qeth_get_elements_for_range(
-				(addr_t)skb->data + data_offset,
-				(addr_t)skb->data + skb_headlen(skb)) +
-			qeth_get_elements_for_frags(skb);
+	addr_t end = (addr_t)skb->data + skb_headlen(skb);
+	int elements = qeth_get_elements_for_frags(skb);
+	addr_t start = (addr_t)skb->data + data_offset;
+
+	if (start != end)
+		elements += qeth_get_elements_for_range(start, end);
 
 	if ((elements + extra_elems) > QETH_MAX_BUFFER_ELEMENTS(card)) {
 		QETH_DBF_MESSAGE(2, "Invalid size of IP packet "

drivers/s390/net/qeth_l3_main.c

@@ -2450,11 +2450,12 @@ static void qeth_tso_fill_header(struct qeth_card *card,
 static int qeth_l3_get_elements_no_tso(struct qeth_card *card,
 			struct sk_buff *skb, int extra_elems)
 {
-	addr_t tcpdptr = (addr_t)tcp_hdr(skb) + tcp_hdrlen(skb);
-	int elements = qeth_get_elements_for_range(
-				tcpdptr,
-				(addr_t)skb->data + skb_headlen(skb)) +
-				qeth_get_elements_for_frags(skb);
+	addr_t start = (addr_t)tcp_hdr(skb) + tcp_hdrlen(skb);
+	addr_t end = (addr_t)skb->data + skb_headlen(skb);
+	int elements = qeth_get_elements_for_frags(skb);
+
+	if (start != end)
+		elements += qeth_get_elements_for_range(start, end);
 
 	if ((elements + extra_elems) > QETH_MAX_BUFFER_ELEMENTS(card)) {
 		QETH_DBF_MESSAGE(2,