dm flakey: fix reads to be issued if drop_writes configured

commit 299f6230bc6d0ccd5f95bb0fb865d80a9c7d5ccc upstream.

v4.8-rc3 commit 99f3c90d0d ("dm flakey: error READ bios during the
down_interval") overlooked the 'drop_writes' feature, which is meant to
allow reads to be issued rather than errored, during the down_interval.

Fixes: 99f3c90d0d ("dm flakey: error READ bios during the down_interval")
Reported-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>