ReplicatedPG::finish_promote: fix snap promote head snaps

If the snaps vector is: 10=[9,5,2]:[4]+head, the backing pool's snaps
vector is 3=[2]:[]+head, and we request clone 4 from the backing pool,
the backing pool will send us head with an empty results->snaps vector.
Actually, clone 4 should be trimmed, but the cache pool does not know
that.  Thus, we should construct an empty snaps vector for that clone.

PartialFix: #11296
Backport: firefly, hammer
Signed-off-by: Samuel Just <sjust@redhat.com>
(cherry picked from commit a45a698)

src/osd/ReplicatedPG.cc

@@ -6642,11 +6642,10 @@ void ReplicatedPG::finish_promote(int r, CopyResults *results,
 	vector<snapid_t>::iterator p = snapset.snaps.begin();
 	while (p != snapset.snaps.end() && *p > soid.snap)
 	  ++p;
-	assert(p != snapset.snaps.end());
-	do {
+	while (p != snapset.snaps.end() && *p > results->snap_seq) {
 	  tctx->new_obs.oi.snaps.push_back(*p);
 	  ++p;
-	} while (p != snapset.snaps.end() && *p > results->snap_seq);
+	}
       }
       dout(20) << __func__ << " snaps " << tctx->new_obs.oi.snaps << dendl;
       assert(!tctx->new_obs.oi.snaps.empty());