osd: requeue blocked op before flush it was blocked on

osd: requeue blocked op before flush it was blocked on

If we have request A (say, cache-flush) that blocks things, and then
request B that gets blocked on it, and we have an interval change, then we
need to requeue B first, then A, so that the resulting queue will keep
A before B and preserve the order.

This was observed on this firefly run:

  ubuntu@teuthology:/a/sage-2015-01-09_21:43:43-rados-firefly-distro-basic-multi/694675

Backport: giant, firefly
Fixes: #10512
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 11bdfb4131ecac16d4a364d651c6cf5d1d28c702)