dm-flakey: Fix corrupt_bio_byte setup checks

Fix the error_reads mode - it's incompatible with corrupt_bio_byte, but
that's only enabled if corrupt_bio_byte is nonzero.

Cc: Benjamin Marzinski <bmarzins@redhat.com>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: Mike Snitzer <snitzer@kernel.org>
Cc: dm-devel@lists.linux.dev
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Reviewed-by: Benjamin Marzinski <bmarzins@redhat.com>
Fixes: 19da6b2c ("dm-flakey: Clean up parsing messages")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>