Use O_PATH to indicate we want to use the fd for unlinkat() only

The O_PATH flag tells open() that we only need a path reference, not
a file descriptor that we can do I/O through. That should make the
operation a lot cheaper to perform.

Also remove third argument to open(): since we are not creating a file,
this argument has no effect.

Author: mthuurne

initramfs.c

@@ -1,4 +1,5 @@
 #define _BSD_SOURCE
+#define _GNU_SOURCE
 
 #include <sys/mount.h>
 #include <sys/types.h>
@@ -13,6 +14,10 @@
 #define MS_MOVE 8192
 #endif
 
+#ifndef O_PATH
+#define O_PATH 010000000
+#endif
+
 #include "debug.h"
 #include "mininit.h"
 
@@ -74,7 +79,7 @@ const char *mount_boot()
 
 int open_dir_to_clean()
 {
-	int fd = open("/", O_RDONLY, 0);
+	int fd = open("/", O_PATH | O_DIRECTORY);
 	if (fd < 0) {
 		DEBUG("Failed to open '/' before switch: %d\n", errno);
 	}