MD: not clear ->safemode for external metadata array

->safemode should be triggered by mdadm for external metadaa array, otherwise
array's state confuses mdadm.

Fixes: 33182d15(md: always clear ->safemode when md_check_recovery gets the mddev lock.)
Cc: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/md.c

@@ -8656,7 +8656,7 @@ void md_check_recovery(struct mddev *mddev)
 	if (mddev_trylock(mddev)) {
 		int spares = 0;
 
-		if (mddev->safemode == 1)
+		if (!mddev->external && mddev->safemode == 1)
 			mddev->safemode = 0;
 
 		if (mddev->ro) {