dm-flakey: error all IOs when num_features is absent

dm-flakey: error all IOs when num_features is absent

dm-flakey would error all IOs if num_features was 0, but if it was
absent, dm-flakey would never error any IO. Fix this so that no
num_features works the same as num_features set to 0.

Fixes: aa7d7bc99fed7 ("dm flakey: add an "error_reads" option")
Reported-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>