Btrfs: fix crash when mounting raid5 btrfs with missing disks

The reproducer is

$ mkfs.btrfs D1 D2 D3 -mraid5
$ mkfs.ext4 D2 && mkfs.ext4 D3
$ mount D1 /btrfs -odegraded

-------------------

[ 87.672992] ------------[ cut here ]------------
[ 87.673845] kernel BUG at fs/btrfs/raid56.c:1828!
...
[ 87.673845] RIP: 0010:[<ffffffff813efc7e>] [<ffffffff813efc7e>] __raid_recover_end_io+0x4ae/0x4d0
...
[ 87.673845] Call Trace:
[ 87.673845] [<ffffffff8116bbc6>] ? mempool_free+0x36/0xa0
[ 87.673845] [<ffffffff813f0255>] raid_recover_end_io+0x75/0xa0
[ 87.673845] [<ffffffff81447c5b>] bio_endio+0x5b/0xa0
[ 87.673845] [<ffffffff81447cb2>] bio_endio_nodec+0x12/0x20
[ 87.673845] [<ffffffff81374621>] end_workqueue_fn+0x41/0x50
[ 87.673845] [<ffffffff813ad2aa>] normal_work_helper+0xca/0x2c0
[ 87.673845] [<ffffffff8108ba2b>] process_one_work+0x1eb/0x530
[ 87.673845] [<ffffffff8108b9c9>] ? process_one_work+0x189/0x530
[ 87.673845] [<ffffffff8108c15b>] worker_thread+0x11b/0x4f0
[ 87.673845] [<ffffffff8108c040>] ? rescuer_thread+0x290/0x290
[ 87.673845] [<ffffffff810939c4>] kthread+0xe4/0x100
[ 87.673845] [<ffffffff810938e0>] ? kthread_create_on_node+0x220/0x220
[ 87.673845] [<ffffffff817e7c7c>] ret_from_fork+0x7c/0xb0
[ 87.673845] [<ffffffff810938e0>] ? kthread_create_on_node+0x220/0x220

-------------------

It's because that we miscalculate @rbio->bbio->error so that it doesn't
reach maximum of tolerable errors while it should have.

Signed-off-by: Liu Bo <[e-mail]>
Tested-by: Satoru Takeuchi<[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/raid56.c

···
 	 * pages are going to be uptodate.
 	 */
 	for (stripe = 0; stripe < bbio->num_stripes; stripe++) {
-		if (rbio->faila == stripe ||
-		    rbio->failb == stripe)
+		if (rbio->faila == stripe || rbio->failb == stripe) {
+			atomic_inc(&rbio->bbio->error);
 			continue;
+		}
 
 		for (pagenr = 0; pagenr < nr_pages; pagenr++) {
 			struct page *p;