sys/xen: Use __printflike() instead of format(printf)

The __printflike macro sets the format to freebsd_kprintf which recent
clang understands and warns about. Fixes the following error:
`passing 'printf' format string where 'freebsd_kprintf' format string is expected [-Werror,-Wformat]`

MFC after:	1 week

sys/dev/xen/blkback/blkback.c

@@ -152,7 +152,7 @@ struct xbb_softc;
 struct xbb_xen_req;
 
 static void xbb_attach_failed(struct xbb_softc *xbb, int err, const char *fmt,
-			      ...) __attribute__((format(printf, 3, 4)));
+			      ...) __printflike(3, 4);
 static int  xbb_shutdown(struct xbb_softc *xbb);
 
 /*------------------------------ Data Structures -----------------------------*/

sys/xen/xen_intr.h

@@ -182,7 +182,7 @@ void xen_intr_unbind(xen_intr_handle_t *handle);
  */
 int
 xen_intr_describe(xen_intr_handle_t port_handle, const char *fmt, ...)
-	__attribute__((format(printf, 2, 3)));
+	__printflike(2, 3);
 
 /**
  * Signal the remote peer of an interrupt source associated with an

sys/xen/xenbus/xenbusvar.h

@@ -151,7 +151,7 @@ int xenbus_grant_ring(device_t dev, unsigned long ring_mfn, grant_ref_t *refp);
  *             printf arguments.
  */
 void xenbus_dev_error(device_t dev, int err, const char *fmt, ...)
-	__attribute__((format(printf, 3, 4)));
+	__printflike(3, 4);
 
 /**
  * va_list version of xenbus_dev_error().
@@ -162,7 +162,7 @@ void xenbus_dev_error(device_t dev, int err, const char *fmt, ...)
  * \param ap   Va_list of printf arguments.
  */
 void xenbus_dev_verror(device_t dev, int err, const char *fmt, va_list ap)
-	__attribute__((format(printf, 3, 0)));
+	__printflike(3, 0);
 
 /**
  * Equivalent to xenbus_dev_error(), followed by
@@ -174,7 +174,7 @@ void xenbus_dev_verror(device_t dev, int err, const char *fmt, va_list ap)
  *             printf arguments.
  */
 void xenbus_dev_fatal(device_t dev, int err, const char *fmt, ...)
-	__attribute__((format(printf, 3, 4)));
+	__printflike(3, 4);
 
 /**
  * va_list version of xenbus_dev_fatal().
@@ -185,7 +185,7 @@ void xenbus_dev_fatal(device_t dev, int err, const char *fmt, ...)
  * \param ap   Va_list of printf arguments.
  */
 void xenbus_dev_vfatal(device_t dev, int err, const char *fmt, va_list)
-	__attribute__((format(printf, 3, 0)));
+	__printflike(3, 0);
 
 /**
  * Convert a member of the xenbus_state enum into an ASCII string.

sys/xen/xenstore/xenstorevar.h

@@ -270,7 +270,7 @@ int xs_scanf(struct xs_transaction t,
  */
 int xs_printf(struct xs_transaction t, const char *dir,
     const char *node, const char *fmt, ...)
-    __attribute__((format(printf, 4, 5)));
+    __printflike(4, 5);
 
 /**
  * va_list version of xenbus_printf().