btrfs: adjust dirty_metadata_bytes after writeback failure of extent buffer

Before, if a eb failed to write out, we would end up triggering a
BUG_ON(). As of f4340622 ("btrfs: extent_io: Move the BUG_ON() in
flush_write_bio() one level up"), we no longer BUG_ON(), so we should
make life consistent and add back the unwritten bytes to
dirty_metadata_bytes.

Fixes: f4340622

 ("btrfs: extent_io: Move the BUG_ON() in flush_write_bio() one level up")
CC: stable@vger.kernel.org # 5.2+
Reviewed-by: Filipe Manana <fdmanana@kernel.org>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: David Sterba <dsterba@suse.com>