dm mirror: fix read error on recovery after default leg failure

dm mirror: fix read error on recovery after default leg failure

If a default leg has failed, any read will cause a new operational
default leg to be selected and the read is resubmitted.  But until now
the read will return failure even though it was successful due to
resubmission.  The reason for this is bio->bi_error was not being
cleared before resubmitting the bio.

Fix by clearing bio->bi_error before resubmission.

Fixes: 4246a0b63bd8 ("block: add a bi_error field to struct bio")
Cc: stable@vger.kernel.org # 4.3+
Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>