nfs: fix nfs4d readlink truncated packet

XDR requires 4-byte alignment; nfs4d READLINK reply writes out the padding,
but truncates the packet to the padding-less size.

Fix by taking the padding into consideration when truncating the packet.

Symptoms:

	# ll /mnt/
	ls: cannot read symbolic link /mnt/test: Input/output error
	total 4
	-rw-r--r--. 1 root root  0 Jun 14 01:21 123456
	lrwxrwxrwx. 1 root root  6 Jul  2 03:33 test
	drwxr-xr-x. 1 root root  0 Jul  2 23:50 tmp
	drwxr-xr-x. 1 root root 60 Jul  2 23:44 tree

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Fixes: 476a7b1f4b2c (nfsd4: don't treat readlink like a zero-copy operation)
Reviewed-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4xdr.c

@@ -3278,7 +3278,7 @@ nfsd4_encode_readlink(struct nfsd4_compoundres *resp, __be32 nfserr, struct nfsd
 
 	wire_count = htonl(maxcount);
 	write_bytes_to_xdr_buf(xdr->buf, length_offset, &wire_count, 4);
-	xdr_truncate_encode(xdr, length_offset + 4 + maxcount);
+	xdr_truncate_encode(xdr, length_offset + 4 + ALIGN(maxcount, 4));
 	if (maxcount & 3)
 		write_bytes_to_xdr_buf(xdr->buf, length_offset + 4 + maxcount,
 						&zero, 4 - (maxcount&3));