CLEANUP: hlua: stop using variable name "s" alternately for hlua_txn and hlua_smp

CLEANUP: hlua: stop using variable name "s" alternately for hlua_txn and hlua_smp

hlua_run_sample_fetch() uses "struct hlua_smp *s" which starts to become
confusing when "s->s" is used, then hlua_txn_close() uses this for struct
hlua_txn with the same "s->s" everywhere. Let's uniformize everything with
htxn and hsmp as in other places.