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