btrfs: Do not use bio->bi_bdev after submission

The block layer modifies bio->bi_bdev and bio->bi_sector while working on
the bio, they do _not_ come back unmodified in the completion callback.

To call add_page, we need at least some bi_bdev set, which is why the code
was working, previously. With this patch, we use the latest_bdev from
fsinfo instead of the leftover in the bio. This gives us the possibility to
use the bi_bdev field for another purpose.
Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>

fs/btrfs/inode.c

@@ -1915,7 +1915,7 @@ static int btrfs_io_failed_hook(struct bio *failed_bio,
 	bio->bi_private = state;
 	bio->bi_end_io = failed_bio->bi_end_io;
 	bio->bi_sector = failrec->logical >> 9;
-	bio->bi_bdev = failed_bio->bi_bdev;
+	bio->bi_bdev = BTRFS_I(inode)->root->fs_info->fs_devices->latest_bdev;
 	bio->bi_size = 0;
 
 	bio_add_page(bio, page, failrec->len, start - page_offset(page));