make dkprintf() evaluate its parameters always

Parameters of dkprintf() should be evaluated even if dkprintf() is
disabled.  Because this enables to find expression of parameter obsolete
and to avoid unnecessary compiler warnings such as "unused variable".

kernel/devobj.c

@@ -33,7 +33,7 @@
 #include <string.h>
 #include <syscall.h>
 
-#define	dkprintf(...)
+#define	dkprintf(...)	do { if (0) kprintf(__VA_ARGS__); } while (0)
 #define	ekprintf(...)	kprintf(__VA_ARGS__)
 
 struct devobj {