fix: reset pooled NSScanner RPC responses to avoid stale usage data

NSScanner responses stream either a dataUsageEntry update or the
final dataUsageCache. The grid framework recycles stream responses
into a pool without resetting them, and the handler set only one
field per response - so a recycled response still carried the Final
(or Update) payload of an earlier scan. A remote node reading such a
response accepted the stale final cache, corrupting bucket usage
data across scanner cycles.

Honor Recycler types when pooling stream responses, implement it for
nsScannerResp, and clear the unset field at the response
construction sites for defense in depth.

Signed-off-by: Julien Laurenceau <julien.laurenceau@pepitedata.com>

cmd/storage-rest-common.go

@@ -83,3 +83,13 @@ type nsScannerResp struct {
 	Update *dataUsageEntry `msg:"u"`
 	Final  *dataUsageCache `msg:"f"`
 }
+
+// Recycle resets the response, so that recycled responses cannot
+// leak stale scanner data into a subsequent scan.
+func (r *nsScannerResp) Recycle() {
+	if r == nil {
+		return
+	}
+	r.Update = nil
+	r.Final = nil
+}