log: add pr_err_once

This function prints an error message only once.
[xemul: change ({}) to do {} while (0)]
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/log.h

@@ -29,6 +29,14 @@ extern void print_on_level(unsigned int loglevel, const char *format, ...)
 #define pr_info(fmt, ...)	\
 	print_on_level(LOG_INFO,	LOG_PREFIX fmt, ##__VA_ARGS__)
 
+#define pr_err_once(fmt, ...) do {	\
+	static bool __printed;		\
+	if (!__printed) {		\
+		print_on_level(LOG_ERROR,	"Error (%s:%d): " LOG_PREFIX fmt, __FILE__, __LINE__, ##__VA_ARGS__); \
+		__printed = 1;		\
+	}				\
+} while (0)
+
 #define pr_err(fmt, ...)	\
 	print_on_level(LOG_ERROR,	"Error (%s:%d): " LOG_PREFIX fmt, __FILE__, __LINE__, ##__VA_ARGS__)