nfsd: fix oops on unsupported operation

I'm hitting the BUG in nfsd4_max_reply() at fs/nfsd/nfs4proc.c:2495 when
client sends an operation the server doesn't support.

in nfsd4_max_reply() it checks for NULL rsize_bop but a non-supported
operation wouldn't have that set.

Cc: Kinglong Mee <kinglongmee@gmail.com>
Fixes: 2282cd2c05e2 "NFSD: Get response size before operation..."
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4proc.c

@@ -2489,7 +2489,7 @@ bool nfsd4_spo_must_allow(struct svc_rqst *rqstp)
 
 int nfsd4_max_reply(struct svc_rqst *rqstp, struct nfsd4_op *op)
 {
-	if (op->opnum == OP_ILLEGAL)
+	if (op->opnum == OP_ILLEGAL || op->status == nfserr_notsupp)
 		return op_encode_hdr_size * sizeof(__be32);
 
 	BUG_ON(OPDESC(op)->op_rsize_bop == NULL);