dosfsck: Add custom exit code for the case where the FS is read only

Signed-off-by: San Mehat <san@google.com>
diff --git a/src/io.c b/src/io.c
index 0ab0bbf..e1b07fc 100644
--- a/src/io.c
+++ b/src/io.c
@@ -76,8 +76,10 @@
 {
     struct stat stbuf;
 
-    if ((fd = open(path,rw ? O_RDWR : O_RDONLY)) < 0)
-	pdie("open %s",path);
+    if ((fd = open(path,rw ? O_RDWR : O_RDONLY)) < 0) {
+        perror("open");
+        exit(6);   
+    }
     changes = last = NULL;
     did_change = 0;