osd/ReplicatedPG: refuse to flush when older dirty clones are present

osd/ReplicatedPG: refuse to flush when older dirty clones are present

If the next oldest clone is dirty, we cannot flush.  That is, we must
always flush starting with the oldest dirty clone.

Note that we can never have a sequence like dirty -> clean -> dirty,
because clones are only dirty on creation, are created in order, and cannot
be flushed (cleaned) out of order.  Thus checking the previous clone is
sufficient (and thankfully cheap).

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