nfsd: fix return of nfs4_acl_write_who

AFAICT, the only way to hit this error is to pass this function a bogus
"who" value. In that case, we probably don't want to return -1 as that
could get sent back to the client. Turn this into nfserr_serverfault,
which is a more appropriate error for a server bug like this.
Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4acl.c

@@ -935,5 +935,5 @@ __be32 nfs4_acl_write_who(struct xdr_stream *xdr, int who)
 		return 0;
 	}
 	WARN_ON_ONCE(1);
-	return -1;
+	return nfserr_serverfault;
 }