nfsd: fix b0rken error value for setattr on read-only mount

commit 96f6f985 upstream.

..._want_write() returns -EROFS on failure, _not_ an NFS error value.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/nfsd/nfs4proc.c

@@ -827,6 +827,7 @@ nfsd4_setattr(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	      struct nfsd4_setattr *setattr)
 {
 	__be32 status = nfs_ok;
+	int err;
 
 	if (setattr->sa_iattr.ia_valid & ATTR_SIZE) {
 		nfs4_lock_state();
@@ -838,9 +839,9 @@ nfsd4_setattr(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 			return status;
 		}
 	}
-	status = mnt_want_write(cstate->current_fh.fh_export->ex_path.mnt);
-	if (status)
-		return status;
+	err = mnt_want_write(cstate->current_fh.fh_export->ex_path.mnt);
+	if (err)
+		return nfserrno(err);
 	status = nfs_ok;
 
 	status = check_attr_support(rqstp, cstate, setattr->sa_bmval,