disable iSCSI ping while ping_timeout=0

[Chen Wen]

Hi

I install VMWare ESXi 5.5 in FreeBSD native iSCSI lun and boot from san(iPXE).
I found NOP-out will make iPXE disconnect this session. 
iPXE’s wiki page says that you can disable NOP-out when you using LIO.

I suggest that we do the same thing while user sets ping_timeout=0.

> static void
> cfiscsi_callout(void *context)
> {
>         struct icl_pdu *cp;
>         struct iscsi_bhs_nop_in *bhsni;
>         struct cfiscsi_session *cs;
> 
>         cs = context;
> 
>         if (cs->cs_terminating)
>                 return;
> 
>         callout_schedule(&cs->cs_callout, 1 * hz);
> 
        if (ping_timeout == 0)
                return;
> 
>         CFISCSI_SESSION_LOCK(cs);
>         cs->cs_timeout++;
>         CFISCSI_SESSION_UNLOCK(cs);