nfsd4: more precise nfsd4_max_reply

It will turn out to be useful to have a more accurate estimate of reply
size; so, piggyback on the existing op reply-size estimators.

Also move nfsd4_max_reply to nfs4proc.c to get easier access to struct
nfsd4_operation and friends.  (Thanks to Christoph Hellwig for pointing
out that simplification.)

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
diff --git a/fs/nfsd/xdr4.h b/fs/nfsd/xdr4.h
index ee9ffdc..41e5229 100644
--- a/fs/nfsd/xdr4.h
+++ b/fs/nfsd/xdr4.h
@@ -536,6 +536,7 @@
 	return argp->opcnt == resp->opcnt;
 }
 
+int nfsd4_max_reply(struct svc_rqst *rqstp, struct nfsd4_op *op);
 void warn_on_nonidempotent_op(struct nfsd4_op *op);
 
 #define NFS4_SVC_XDRSIZE		sizeof(struct nfsd4_compoundargs)