nfs: cover ->migratepage with CONFIG_MIGRATION

It will be more clean to use CONFIG_MIGRATION to cover nfs' private
.migratepage in nfs_file_aops like we do in other part of nfs
operations.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>