NFSD: fix bug for readdir of pseudofs

Commit 561f0ed4 (nfsd4: allow large readdirs) introduces a bug
about readdir the root of pseudofs.

Call xdr_truncate_encode() revert encoded name when skipping.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>