dm mirror: use all available legs on multiple failures

When any leg(s) have failed, any read will cause a new operational
default leg to be selected and the read is resubmitted to it.  If that
new default leg fails the read too, no other still accessible legs are
used to resubmit the read again -- thus failing the io.

Fix by allowing the read to get resubmitted until all operational legs
have been exhausted.  Also, remove any details.bi_dev use as a flag.

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>