Manage: when re-adding, do check avail size if ->sb cannot be found.

avail_size1 requires ->sb, so we must only call it if ->sb
was loaded.

If ->sb wasn't loaded, then we are only proceding on the basis that
the kernel might be able to work something out - we don't need to
do any tests on size.

Reported-by: Christoffer Hammarström <christoffer.hammarstrom@linuxgods.com>
Signed-off-by: NeilBrown <neilb@suse.de>
URL: https://bugs.debian.org/784874

Manage.c

@@ -781,7 +781,8 @@ int Manage_add(int fd, int tfd, struct mddev_dev *dv,
 		}
 
 		/* Make sure device is large enough */
-		if (tst->ss->avail_size(tst, ldsize/512, INVALID_SECTORS) <
+		if (tst->sb &&
+		    tst->ss->avail_size(tst, ldsize/512, INVALID_SECTORS) <
 		    array_size) {
 			if (dv->disposition == 'M')
 				return 0;