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>
diff --git a/fs/nfs/file.c b/fs/nfs/file.c
index ca699dd..580d1c5 100644
--- a/fs/nfs/file.c
+++ b/fs/nfs/file.c
@@ -543,7 +543,9 @@
 	.invalidatepage = nfs_invalidate_page,
 	.releasepage = nfs_release_page,
 	.direct_IO = nfs_direct_IO,
+#ifdef CONFIG_MIGRATION
 	.migratepage = nfs_migrate_page,
+#endif
 	.launder_page = nfs_launder_page,
 	.is_dirty_writeback = nfs_check_dirty_writeback,
 	.error_remove_page = generic_error_remove_page,