atapi: classify read_cd as conditionally returning data

For the purposes of byte_count_limit verification, add a new flag that
identifies read_cd as sometimes returning data, then check the BCL in
its command handler after we know that it will indeed return data.

Reported-by: Hervé Poussineau <hpoussin@reactos.org>
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 1477970211-25754-2-git-send-email-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>
