dm raid: fix false positive for requeue needed during reshape

An empty flush doesn't have a payload, so it should never be looked at
when considering to possibly requeue a bio for the case when a reshape
is in progress.

Fixes: 9dbd1aa3

 ("dm raid: add reshaping support to the target")
Reported-by: Patrick Plenefisch <simonpatp@gmail.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>