btrfs-progs: add helper to print messages to stderr

Add similar helper to pr_verbose that prints on stderr, for commands
that need to print to stderr based on the set verbosity level.

Signed-off-by: David Sterba <dsterba@suse.com>

common/messages.c

@@ -151,3 +151,21 @@ void error_msg(enum common_error error, const char *msg, ...)
 		fprintf(stderr, PREFIX_ERROR "%s\n", str);
 	}
 }
+
+/*
+ * Print a message according to the global verbosity level - to stderr.
+ *
+ * level: minimum verbose level at which the message will be printed
+ */
+__attribute__ ((format (printf, 2, 3)))
+void pr_stderr(int level, const char *fmt, ...)
+{
+	va_list args;
+
+	if (!should_print(level))
+		return;
+
+	va_start(args, fmt);
+	vfprintf(stderr, fmt, args);
+	va_end(args);
+}

common/messages.h

@@ -130,6 +130,9 @@ void internal_error(const char *fmt, ...);
 __attribute__ ((format (printf, 2, 3)))
 void pr_verbose(int level, const char *fmt, ...);
 
+__attribute__ ((format (printf, 2, 3)))
+void pr_stderr(int level, const char *fmt, ...);
+
 /*
  * Commonly used errors
  */