Fix liveness check for NAS gateway (#7142)

Current master throws '503' unavailable for liveness check
```
~ curl -v http://localhost:9000/minio/health/live
> GET /minio/health/live HTTP/1.1
...
...
< HTTP/1.1 503 Service Unavailable
```

With this fix liveness check returns error appropriately
```
~ curl -v http://localhost:9000/minio/health/live
> GET /minio/health/live HTTP/1.1
...
...
< HTTP/1.1 200 OK
```

cmd/gateway/nas/gateway-nas.go

@@ -17,6 +17,8 @@
 package nas
 
 import (
+	"context"
+
 	"github.com/minio/cli"
 	minio "github.com/minio/minio/cmd"
 	"github.com/minio/minio/pkg/auth"
@@ -121,6 +123,12 @@ func (n *nasObjects) IsListenBucketSupported() bool {
 	return false
 }
 
+func (n *nasObjects) StorageInfo(ctx context.Context) minio.StorageInfo {
+	sinfo := n.ObjectLayer.StorageInfo(ctx)
+	sinfo.Backend.Type = minio.Unknown
+	return sinfo
+}
+
 // nasObjects implements gateway for Minio and S3 compatible object storage servers.
 type nasObjects struct {
 	minio.ObjectLayer