sys/rpc: UNIX auth: Support XDR_FREE

xdr_authunix_parms() does not allocate any auxiliary memory, so we can
simply support XDR_FREE by just returning TRUE.

Although there are currently no callers passing XDR_FREE, this makes us
immune to such a change in a way that doesn't cost more but is more
constructive than a mere KASSERT().

Suggested by:   rmacklem
MFC after:      2 days
Sponsored by:   The FreeBSD Foundation

Author: OlCe2

sys/rpc/authunix_prot.c

@@ -61,6 +61,10 @@ xdr_authunix_parms(XDR *xdrs, uint32_t *time, struct xucred *cred)
 	uint32_t junk;
 	char hostbuf[MAXHOSTNAMELEN];
 
+	if (xdrs->x_op == XDR_FREE)
+		/* This function does not allocate auxiliary memory. */
+		return (TRUE);
+
 	if (xdrs->x_op == XDR_ENCODE) {
 		getcredhostname(NULL, hostbuf, sizeof(hostbuf));
 		namelen = strlen(hostbuf);