util: Ability to ignore errno when opening proc

When run from regular user criu will get EACCES/EPERM from
opening proc, but in some situations criu will now how to
deal with it. So this patch makes it possible not to print
error message in logs for such cases.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Looks-good-to-me: Andrew Vagin <avagin@virtuozzo.com>

include/util.h

@@ -75,11 +75,11 @@ extern int set_proc_fd(int fd);
 
 extern int do_open_proc(pid_t pid, int flags, const char *fmt, ...);
 
-#define __open_proc(pid, flags, fmt, ...)				\
+#define __open_proc(pid, ier, flags, fmt, ...)				\
 	({								\
 		int __fd = do_open_proc(pid, flags,			\
 					fmt, ##__VA_ARGS__);		\
-		if (__fd < 0)						\
+		if (__fd < 0 && (errno != ier))				\
 			pr_perror("Can't open %d/" fmt " on procfs",	\
 					pid, ##__VA_ARGS__);		\
 									\
@@ -88,14 +88,14 @@ extern int do_open_proc(pid_t pid, int flags, const char *fmt, ...);
 
 /* int open_proc(pid_t pid, const char *fmt, ...); */
 #define open_proc(pid, fmt, ...)				\
-	__open_proc(pid, O_RDONLY, fmt, ##__VA_ARGS__)
+	__open_proc(pid, 0, O_RDONLY, fmt, ##__VA_ARGS__)
 
 /* int open_proc_rw(pid_t pid, const char *fmt, ...); */
 #define open_proc_rw(pid, fmt, ...)				\
-	__open_proc(pid, O_RDWR, fmt, ##__VA_ARGS__)
+	__open_proc(pid, 0, O_RDWR, fmt, ##__VA_ARGS__)
 
 #define open_proc_path(pid, fmt, ...)				\
-	__open_proc(pid, O_PATH, fmt, ##__VA_ARGS__)
+	__open_proc(pid, 0, O_PATH, fmt, ##__VA_ARGS__)
 
 /* DIR *opendir_proc(pid_t pid, const char *fmt, ...); */
 #define opendir_proc(pid, fmt, ...)					\