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/super.c b/fs/nilfs2/super.c
index 7fe497e..86e3c00 100644
--- a/fs/nilfs2/super.c
+++ b/fs/nilfs2/super.c
@@ -71,6 +71,22 @@
 static int nilfs_setup_super(struct super_block *sb, int is_mount);
 static int nilfs_remount(struct super_block *sb, int *flags, char *data);
 
+void __nilfs_msg(struct super_block *sb, const char *level, const char *fmt,
+		 ...)
+{
+	struct va_format vaf;
+	va_list args;
+
+	va_start(args, fmt);
+	vaf.fmt = fmt;
+	vaf.va = &args;
+	if (sb)
+		printk("%sNILFS (%s): %pV\n", level, sb->s_id, &vaf);
+	else
+		printk("%sNILFS: %pV\n", level, &vaf);
+	va_end(args);
+}
+
 static void nilfs_set_error(struct super_block *sb)
 {
 	struct the_nilfs *nilfs = sb->s_fs_info;