xen-pciback: Add name prefix to global 'permissive' variable

The variable for the 'permissive' module parameter used to be static
but was recently changed to be extern.  This puts it in the kernel
global namespace if the driver is built-in, so its name should begin
with a prefix identifying the driver.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Fixes: af6fc858a35b ("xen-pciback: limit guest control of command register")
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
diff --git a/drivers/xen/xen-pciback/conf_space_header.c b/drivers/xen/xen-pciback/conf_space_header.c
index c2260a0..ad3d17d 100644
--- a/drivers/xen/xen-pciback/conf_space_header.c
+++ b/drivers/xen/xen-pciback/conf_space_header.c
@@ -118,7 +118,7 @@
 
 	cmd->val = value;
 
-	if (!permissive && (!dev_data || !dev_data->permissive))
+	if (!xen_pcibk_permissive && (!dev_data || !dev_data->permissive))
 		return 0;
 
 	/* Only allow the guest to control certain bits. */