osd/ReplicatedPG: check clones for degraded

We check whether the head is degraded, and we check whether a clone is 
unreadable, but in the case where we have a cache op on a degraded object, we
don't check.  That leads to an assert when the repop hits the replica and the
object is in the peer's missing set.

Fix this by adding a check on the clone when write_ordered is true.  Note 
that checking write_ordered is better than whether it is a cache op because 
we want to preserve write ordering even for reads that are flagged by the 
client.

Fixes: #8048 Signed-off-by: Sage Weil <sage@inktank.com>