ext4: address a benign compiler warning

When !defined(CONFIG_EXT4_DEBUG), mb_debug() should be defined as a
no_printk() statement instead of an empty statement in order to suppress
the following compiler warning:

fs/ext4/mballoc.c: In function ‘ext4_mb_cleanup_pa’:
fs/ext4/mballoc.c:2659:47: warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]
   mb_debug(1, "mballoc: %u PAs left\n", count);
Signed-off-by: Patrick Palka <patrick@parcs.ath.cx>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/mballoc.h

@@ -48,7 +48,7 @@ extern ushort ext4_mballoc_debug;
 		}							\
 	} while (0)
 #else
-#define mb_debug(n, fmt, a...)
+#define mb_debug(n, fmt, a...)		no_printk(fmt, ## a)
 #endif
 
 #define EXT4_MB_HISTORY_ALLOC		1	/* allocation */