mount: invalidate kernel cache when mounted to a filer path

fix https://github.com/chrislusf/seaweedfs/issues/1752#issuecomment-768178422

weed/filesys/meta_cache/meta_cache.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"os"
+	"strings"
 	"sync"
 
 	"github.com/chrislusf/seaweedfs/weed/filer"
@@ -29,7 +30,12 @@ func NewMetaCache(dbFolder string, baseDir util.FullPath, uidGidMapper *UidGidMa
 		localStore:      openMetaStore(dbFolder),
 		visitedBoundary: bounded_tree.NewBoundedTree(baseDir),
 		uidGidMapper:    uidGidMapper,
-		invalidateFunc:  invalidateFunc,
+		invalidateFunc: func(fullpath util.FullPath) {
+			if baseDir != "/" && strings.HasPrefix(string(fullpath), string(baseDir)) {
+				fullpath = fullpath[len(baseDir):]
+			}
+			invalidateFunc(fullpath)
+		},
 	}
 }