NFSD: fix bug for readdir of pseudofs

Commit 561f0ed498ca (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>

fs/nfsd/nfs4xdr.c

@@ -2687,6 +2687,7 @@ nfsd4_encode_dirent(void *ccdv, const char *name, int namlen,
 		nfserr = nfserr_toosmall;
 		goto fail;
 	case nfserr_noent:
+		xdr_truncate_encode(xdr, start_offset);
 		goto skip_entry;
 	default:
 		/*