NTB: Fix oops in debugfs when transport is half-up

When the remote side is not up, we do not have all the context for the
transport, and that causes NULL ptr access. Have the debugfs reads check
to see if transport is up before we make access.
Signed-off-by: NDave Jiang <dave.jiang@intel.com>
Signed-off-by: NJon Mason <jdmason@kudzu.us>

drivers/ntb/ntb_transport.c

@@ -439,13 +439,17 @@ static ssize_t debugfs_read(struct file *filp, char __user *ubuf, size_t count,
 	char *buf;
 	ssize_t ret, out_offset, out_count;
 
+	qp = filp->private_data;
+
+	if (!qp || !qp->link_is_up)
+		return 0;
+
 	out_count = 1000;
 
 	buf = kmalloc(out_count, GFP_KERNEL);
 	if (!buf)
 		return -ENOMEM;
 
-	qp = filp->private_data;
 	out_offset = 0;
 	out_offset += snprintf(buf + out_offset, out_count - out_offset,
 			       "NTB QP stats\n");