cephfs: make fsname as optional for static PVC

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

fixes: ceph#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
 	}