cephfs: make fsname as optional for static PVC

consider fsName optional for static volume
as it is not required to be set during mount
operation with fuse and kernel client.

fixes: #4311

Signed-off-by: Madhu Rajanna <madhupr007@gmail.com>

internal/cephfs/store/volumeoptions.go

@@ -697,7 +697,7 @@ func NewVolumeOptionsFromStaticVolume(
 		return nil, nil, err
 	}
 
-	if err = extractOption(&opts.FsName, "fsName", options); err != nil {
+	if err = extractOptionalOption(&opts.FsName, "fsName", options); err != nil {
 		return nil, nil, err
 	}