os/bluestore: avoid extra dev flush on single device when all io is deferred

os/bluestore: avoid extra dev flush on single device when all io is deferred

If we have no non-deferred IO to flush, and we are running bluefs on a
single shared device, then we can rely on the bluefs flush to make our
current batch of deferred ios stable.

Separate deferred into a "done" and "stable" list.  If we do sync, put
everything from "done" onto "stable".  Otherwise, after we do our kv
commit via bluefs, move "done" to "stable" then.

Signed-off-by: Sage Weil <sage@redhat.com>