nfsd: export proper maximum file size to the client

I noticed that we export a way to high value for the maxfilesize
attribute when debugging a client issue.  The issue didn't turn
out to be related to it, but I think we should export it, so that
clients can limit what write sizes they accept before hitting
the server.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4xdr.c

@@ -2391,7 +2391,7 @@ nfsd4_encode_fattr(struct svc_fh *fhp, struct svc_export *exp,
 	if (bmval0 & FATTR4_WORD0_MAXFILESIZE) {
 		if ((buflen -= 8) < 0)
 			goto out_resource;
-		WRITE64(~(u64)0);
+		WRITE64(exp->ex_path.mnt->mnt_sb->s_maxbytes);
 	}
 	if (bmval0 & FATTR4_WORD0_MAXLINK) {
 		if ((buflen -= 4) < 0)