md/raid10: reset the 'first' at the end of loop

We need to set "first = 0' at the end of rdev_for_each
loop, so we can get the array's min_offset_diff correctly
otherwise min_offset_diff just means the last rdev's
offset diff.
Suggested-by: NNeilBrown <neilb@suse.com>
Signed-off-by: NGuoqing Jiang <gqjiang@suse.com>
Reviewed-by: NNeilBrown <neilb@suse.com>
Signed-off-by: NShaohua Li <shli@fb.com>

drivers/md/raid10.c

@@ -3769,6 +3769,7 @@ static int raid10_run(struct mddev *mddev)
 
 		if (blk_queue_discard(bdev_get_queue(rdev->bdev)))
 			discard_supported = true;
+		first = 0;
 	}
 
 	if (mddev->queue) {
@@ -4172,6 +4173,7 @@ static int raid10_start_reshape(struct mddev *mddev)
 			if (first || diff < min_offset_diff)
 				min_offset_diff = diff;
 		}
+		first = 0;
 	}
 
 	if (max(before_length, after_length) > min_offset_diff)