nfsd4: teach encoders to handle reserve_space failures

We've tried to prevent running out of space with COMPOUND_SLACK_SPACE
and special checking in those operations (getattr) whose result can vary
enormously.

However:
	- COMPOUND_SLACK_SPACE may be difficult to maintain as we add
	  more protocol.
	- BUG_ON or page faulting on failure seems overly fragile.
	- Especially in the 4.1 case, we prefer not to fail compounds
	  just because the returned result came *close* to session
	  limits.  (Though perfect enforcement here may be difficult.)
	- I'd prefer encoding to be uniform for all encoders instead of
	  having special exceptions for encoders containing, for
	  example, attributes.
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4proc.c

@@ -1400,7 +1400,7 @@ nfsd4_proc_compound(struct svc_rqst *rqstp,
 		}
 		if (op->status == nfserr_replay_me) {
 			op->replay = &cstate->replay_owner->so_replay;
-			nfsd4_encode_replay(resp, op);
+			nfsd4_encode_replay(&resp->xdr, op);
 			status = op->status = op->replay->rp_status;
 		} else {
 			nfsd4_encode_operation(resp, op);

fs/nfsd/xdr4.h

@@ -563,7 +563,7 @@ int nfs4svc_encode_compoundres(struct svc_rqst *, __be32 *,
 		struct nfsd4_compoundres *);
 __be32 nfsd4_check_resp_size(struct nfsd4_compoundres *, u32);
 void nfsd4_encode_operation(struct nfsd4_compoundres *, struct nfsd4_op *);
-void nfsd4_encode_replay(struct nfsd4_compoundres *resp, struct nfsd4_op *op);
+void nfsd4_encode_replay(struct xdr_stream *xdr, struct nfsd4_op *op);
 __be32 nfsd4_encode_fattr_to_buf(__be32 **p, int words,
 		struct svc_fh *fhp, struct svc_export *exp,
 		struct dentry *dentry,