kernel/cred.c: remove unnecessary kdebug atomic reads

Commit e0e817392b9a ("CRED: Add some configurable debugging [try #6]")
added the kdebug mechanism to this file back in 2009.

The kdebug macro calls no_printk which always evaluates arguments.

Most of the kdebug uses have an unnecessary call of
	atomic_read(&cred->usage)

Make the kdebug macro do nothing by defining it with
	do { if (0) no_printk(...); } while (0)
when not enabled.

$ size kernel/cred.o* (defconfig x86-64)
   text	   data	    bss	    dec	    hex	filename
   2748	    336	      8	   3092	    c14	kernel/cred.o.new
   2788	    336	      8	   3132	    c3c	kernel/cred.o.old

Miscellanea:
o Neaten the #define kdebug macros while there

Signed-off-by: Joe Perches <joe@perches.com>
Cc: David Howells <dhowells@redhat.com>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/cred.c

@@ -20,11 +20,16 @@
 #include <linux/cn_proc.h>
 
 #if 0
-#define kdebug(FMT, ...) \
-	printk("[%-5.5s%5u] "FMT"\n", current->comm, current->pid ,##__VA_ARGS__)
+#define kdebug(FMT, ...)						\
+	printk("[%-5.5s%5u] " FMT "\n",					\
+	       current->comm, current->pid, ##__VA_ARGS__)
 #else
-#define kdebug(FMT, ...) \
-	no_printk("[%-5.5s%5u] "FMT"\n", current->comm, current->pid ,##__VA_ARGS__)
+#define kdebug(FMT, ...)						\
+do {									\
+	if (0)								\
+		no_printk("[%-5.5s%5u] " FMT "\n",			\
+			  current->comm, current->pid, ##__VA_ARGS__);	\
+} while (0)
 #endif
 
 static struct kmem_cache *cred_jar;