Btrfs: avoid I/O repair BUG() from btree_read_extent_buffer_pages()

From btree_read_extent_buffer_pages(), currently repair_io_failure()
can be called with mirror_num being zero when submit_one_bio() returned
an error before. This used to cause a BUG_ON(!mirror_num) in
repair_io_failure() and indeed this is not a case that needs the I/O
repair code to rewrite disk blocks.
This commit prevents calling repair_io_failure() in this case and thus
avoids the BUG_ON() and malfunction.
Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/disk-io.c

@@ -407,7 +407,7 @@ static int btree_read_extent_buffer_pages(struct btrfs_root *root,
 			break;
 	}
 
-	if (failed && !ret)
+	if (failed && !ret && failed_mirror)
 		repair_eb_io_failure(root, eb, failed_mirror);
 
 	return ret;