conditionally skip hdfs related code

Makefile

@@ -8,4 +8,4 @@ install:
 	cd weed; go install
 
 full_install:
-	cd weed; go install -tags "elastic gocdk sqlite"
+	cd weed; go install -tags "elastic gocdk sqlite hdfs"

weed/remote_storage/hdfs/doc.go

@@ -0,0 +1,9 @@
+/*
+
+Package hdfs is for remote hdfs storage.
+
+The referenced "github.com/colinmarc/hdfs/v2" library is too big when compiled.
+So this is only compiled in "make full_install".
+
+*/
+package hdfs

weed/remote_storage/hdfs/hdfs_kerberos.go

@@ -1,3 +1,6 @@
+//go:build hdfs
+// +build hdfs
+
 package hdfs
 
 import (

weed/remote_storage/hdfs/hdfs_storage_client.go

@@ -1,3 +1,6 @@
+//go:build hdfs
+// +build hdfs
+
 package hdfs
 
 import (
@@ -7,7 +10,7 @@ import (
 	"github.com/chrislusf/seaweedfs/weed/pb/remote_pb"
 	"github.com/chrislusf/seaweedfs/weed/remote_storage"
 	"github.com/chrislusf/seaweedfs/weed/util"
-	"github.com/colinmarc/hdfs/v2"
+	hdfs "github.com/colinmarc/hdfs/v2"
 	"io"
 	"os"
 	"path"