[SCSI] iscsi class: add session scanning

This just adds iscsi session scanning which works like fc rport scanning.
The future patches will hook the drivers into Mathew Wilcox's async
scanning infrastructure, so userspace does not have to special case
iscsi and so userspace does not have to make a extra special case for
hardware iscsi root scanning.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>