NFSD: fix decoding in nfs4_xdr_dec_cb_getattr

If a client were to send an error to a CB_GETATTR call, the code
erronously continues to try decode past the error code. It ends
up returning BAD_XDR error to the rpc layer and then in turn
trigger a WARN_ONCE in nfsd4_cb_done() function.

Fixes: 6487a13 ("NFSD: add support for CB_GETATTR callback")
Signed-off-by: Olga Kornievskaia <okorniev@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4callback.c

@@ -647,7 +647,7 @@ static int nfs4_xdr_dec_cb_getattr(struct rpc_rqst *rqstp,
 		return status;
 
 	status = decode_cb_op_status(xdr, OP_CB_GETATTR, &cb->cb_status);
-	if (status)
+	if (unlikely(status || cb->cb_seq_status))
 		return status;
 	if (xdr_stream_decode_uint32_array(xdr, bitmap, 3) < 0)
 		return -NFSERR_BAD_XDR;