nilfs2: add nilfs_msg() message interface

Define an own output routine to replace bare use of printk() function.
The output routine is implemented with a macro and a helper function,
which are named nilfs_msg() and __nilfs_msg(), respectively.

__nilfs_msg() formats a message like "NILFS (<device-name>): <message>",
prefixing it with a given log level, and terminates the statement with a
newline.  The "device-name" is optional to make it available in early
stages; it will be omitted if a NULL pointer is passed to super block
instance argument.  nilfs_msg() wraps __nilfs_msg() and is removed if
CONFIG_PRINTK is not set.

Link: http://lkml.kernel.org/r/1464875891-5443-3-git-send-email-konishi.ryusuke@lab.ntt.co.jp
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/nilfs2/nilfs.h b/fs/nilfs2/nilfs.h
index e482c78..b57ce41e 100644
--- a/fs/nilfs2/nilfs.h
+++ b/fs/nilfs2/nilfs.h
@@ -301,6 +301,9 @@
 extern void nilfs_destroy_inode(struct inode *);
 
 extern __printf(3, 4)
+void __nilfs_msg(struct super_block *sb, const char *level,
+		 const char *fmt, ...);
+extern __printf(3, 4)
 void __nilfs_error(struct super_block *sb, const char *function,
 		   const char *fmt, ...);
 extern __printf(3, 4)
@@ -308,11 +311,15 @@
 
 #ifdef CONFIG_PRINTK
 
+#define nilfs_msg(sb, level, fmt, ...)					\
+	__nilfs_msg(sb, level, fmt, ##__VA_ARGS__)
 #define nilfs_error(sb, fmt, ...)					\
 	__nilfs_error(sb, __func__, fmt, ##__VA_ARGS__)
 
 #else
 
+#define nilfs_msg(sb, level, fmt, ...)					\
+	no_printk(fmt, ##__VA_ARGS__)
 #define nilfs_error(sb, fmt, ...)					\
 	do {								\
 		no_printk(fmt, ##__VA_ARGS__);				\