iscsi: don't continue waiting if the root device is present

iscsi: don't continue waiting if the root device is present

dracut waits for every iscsiroot connection to be established
before switching root. This is not necessary in multipath scenarios,
where a single path is usually sufficient to set up the root device,
and where users expect booting to succeed unless all paths are down.

Don't wait for the iscsi portal to start if the root device has
already been found.