fix(memcached): use default branch avoid writing to closed chan (#7833

)

Signed-off-by: Kaviraj <kavirajkanagaraj@gmail.com>

**What this PR does / why we need it**:
Follow up to #7817

This PR uses `default` branch instead of `inputCh <- work` to make sure
we are writing to closed `inputCh`.

Gist is, `default` run [only when none of the branch is
ready](https://go.dev/tour/concurrency/6). which makes more sense rather
than to have `inputCh <- work` (writing to closed channel on the branch
condition)

These can be explained by these two tiny snippets.
* [with `default`](https://go.dev/play/p/-FspbTZd20I)
* [without `default`](https://go.dev/play/p/Ag4WznOaEq0)

**Which issue(s) this PR fixes**:
Fixes #NA

**Special notes for your reviewer**:
We already have test `TestMemcached_fetchKeysBatched` to catch this
behaviour. In fact this test caught this probabilistic behaviour in some
CI failures. [here ](https://drone.grafana.net/grafana/loki/17853/3/4)
and[ here](https://drone.grafana.net/grafana/loki/17854/3/4)

Thanks @DylanGuedes for catching this CI failures.

**Checklist**

Signed-off-by: Kaviraj <kavirajkanagaraj@gmail.com>

pkg/storage/chunk/cache/memcached.go

@@ -172,12 +172,13 @@ func (c *Memcached) fetchKeysBatched(ctx context.Context, keys []string) (found
 			select {
 			case <-c.closed:
 				return
-			case c.inputCh <- &work{
-				keys:     batchKeys,
-				ctx:      ctx,
-				resultCh: resultsCh,
-				batchID:  j,
-			}:
+			default:
+				c.inputCh <- &work{
+					keys:     batchKeys,
+					ctx:      ctx,
+					resultCh: resultsCh,
+					batchID:  j,
+				}
 				j++
 			}
 		}
@@ -199,7 +200,8 @@ func (c *Memcached) fetchKeysBatched(ctx context.Context, keys []string) (found
 		select {
 		case <-c.closed:
 			return
-		case result := <-resultsCh:
+		default:
+			result := <-resultsCh
 			results[result.batchID] = result
 		}
 	}