s390/qeth: fix underestimated count of buffer elements

For a memory range/skb where the last byte falls onto a page boundary
(ie. 'end' is of the form xxx...xxx001), the PFN_UP() part of the
calculation currently doesn't round up to the next PFN due to an
off-by-one error.
Thus qeth believes that the skb occupies one page less than it
actually does, and may select a IO buffer that doesn't have enough spare
buffer elements to fit all of the skb's data.
HW detects this as a malformed buffer descriptor, and raises an
exception which then triggers device recovery.

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

drivers/s390/net/qeth_core.h

@@ -846,7 +846,7 @@ struct qeth_trap_id {
  */
 static inline int qeth_get_elements_for_range(addr_t start, addr_t end)
 {
-	return PFN_UP(end - 1) - PFN_DOWN(start);
+	return PFN_UP(end) - PFN_DOWN(start);
 }
 
 static inline int qeth_get_micros(void)