Previously, iscsi would have a fixed once per second
callout, to either time out the iscsid / login phases,
or to transmit a new iSCSI ping.
In order to reduce the number of callouts while waiting
for iscsid/login, only schedule one callout for when that
has expired.
Schedule the callouts while connected at 1/3 of the ping
timeout interval. This will send Pings only 3 times before
expiring - and at larger intervals, reducing overhead,
especially when larger timeouts are configured.
The expiry of an unresponsive session still happens
as before (after the timeout) but with less burden on the
system.