NFS: Fix up nfs_readdir_inode_mapping_valid()

The check for duplicate readdir cookies should only care if the change
attribute is invalid or the data cache is invalid.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Tested-by: Benjamin Coddington <bcodding@redhat.com>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>