Btrfs: detect corrupted filesystem after write I/O errors

In check-integrity, detect when a superblock is written that points
to blocks that have not been written to disk due to I/O write errors.
Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>

fs/btrfs/check-integrity.c

@@ -37,8 +37,9 @@
  *        the file system was mounted, (i.e., they have been
  *        referenced by the super block) or they have been
  *        written since then and the write completion callback
- *        was called and a FLUSH request to the device where
- *        these blocks are located was received and completed.
+ *        was called and no write error was indicated and a
+ *        FLUSH request to the device where these blocks are
+ *        located was received and completed.
  *    2b. All referenced blocks need to have a generation
  *        number which is equal to the parent's number.
  *
@@ -2601,6 +2602,17 @@ static int btrfsic_check_all_ref_blocks(struct btrfsic_state *state,
 			       (unsigned long long)l->block_ref_to->dev_bytenr,
 			       l->block_ref_to->mirror_num);
 			ret = -1;
+		} else if (l->block_ref_to->iodone_w_error) {
+			printk(KERN_INFO "btrfs: attempt to write superblock"
+			       " which references block %c @%llu (%s/%llu/%d)"
+			       " which has write error!\n",
+			       btrfsic_get_block_type(state, l->block_ref_to),
+			       (unsigned long long)
+			       l->block_ref_to->logical_bytenr,
+			       l->block_ref_to->dev_state->name,
+			       (unsigned long long)l->block_ref_to->dev_bytenr,
+			       l->block_ref_to->mirror_num);
+			ret = -1;
 		} else if (l->parent_generation !=
 			   l->block_ref_to->generation &&
 			   BTRFSIC_GENERATION_UNKNOWN !=