Guard log_printf in LOG macros with a log level check

So that the format arguments will only be evaluated if the log is
enabled by the log level. Allow us to add more expensive logs without
impact performance when they are not enabled.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/log.c

@@ -98,6 +98,10 @@ void log_set_level(struct log *l, int level) {
 	l->log_level = level;
 }
 
+enum log_level log_get_level(const struct log *l) {
+	return l->log_level;
+}
+
 attr_printf(4, 5) void log_printf(struct log *l, int level, const char *func,
                                   const char *fmt, ...) {
 	assert(level <= LOG_LEVEL_FATAL && level >= 0);