open_mddev: open RDONLY if RDWR doesn't work.

If an array is read-only then "mdadm -S"
cannot open it to stop it without this fix.

Signed-off-by: NeilBrown <neilb@suse.de>

mdopen.c

@@ -390,6 +390,8 @@ int create_mddev(char *dev, char *name, int autof, int trustworthy,
 int open_mddev(char *dev, int report_errors)
 {
 	int mdfd = open(dev, O_RDWR);
+	if (mdfd < 0 && errno == EACCES)
+		mdfd = open(dev, O_RDONLY);
 	if (mdfd < 0) {
 		if (report_errors)
 			fprintf(stderr, Name ": error opening %s: %s\n",