dm verity: skip redundant verity_handle_err() on I/O errors

Without FEC, dm-verity won't call verity_handle_err() when I/O fails,
but with FEC enabled, it currently does even if an I/O error has
occurred.

If there is an I/O error and FEC correction fails, return the error
instead of calling verity_handle_err() again.

Suggested-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Akilesh Kailash <akailash@google.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>