md: Allow metadata_version to be updated for externally managed metadata.

For externally managed metadata, the 'metadata_version' sysfs
attribute is really just a channel for user-space programs to
communicate about how the array is being managed.
It can be useful for this to be changed while the array is active.

Normally changes to metadata_version are not permitted while the array
is active.  Change that so that if the metadata is externally managed,
the metadata_version can be changed to a different flavour of external
management.

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

drivers/md/md.c

@@ -2943,7 +2943,13 @@ metadata_store(mddev_t *mddev, const char *buf, size_t len)
 {
 	int major, minor;
 	char *e;
-	if (!list_empty(&mddev->disks))
+	/* Changing the details of 'external' metadata is
+	 * always permitted.  Otherwise there must be
+	 * no devices attached to the array.
+	 */
+	if (mddev->external && strncmp(buf, "external:", 9) == 0)
+		;
+	else if (!list_empty(&mddev->disks))
 		return -EBUSY;
 
 	if (cmd_match(buf, "none")) {