usb: gadget: printer: add configfs support

Add support for configfs interface so that f_printer can be used as a
component of usb gadgets composed with it.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/drivers/usb/gadget/function/u_printer.h b/drivers/usb/gadget/function/u_printer.h
index b2338ca..0e2c49d 100644
--- a/drivers/usb/gadget/function/u_printer.h
+++ b/drivers/usb/gadget/function/u_printer.h
@@ -25,6 +25,13 @@
 	int				minor;
 	char				pnp_string[PNP_STRING_LEN];
 	unsigned			q_len;
+
+	/*
+	 * Protect the data from concurrent access by read/write
+	 * and create symlink/remove symlink
+	 */
+	struct mutex			lock;
+	int				refcnt;
 };
 
 #endif /* U_PRINTER_H */