dm verity fec: fix bufio leaks

Buffers read through dm_bufio_read() were not released in all code paths.

Fixes: a739ff3f ("dm verity: add support for forward error correction")
Cc: stable@vger.kernel.org # v4.5+
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>