[SCSI] iscsi: fix session refcouting

iscsi_tcp and iser cannot be rmmod from the kernel when sessions
are running because session removal is driven from userspace. For
those modules we get a module reference when a session is
created then drop it when the session is removed.

For qla4xxx, they can jsut remove the sessions from the pci remove
function like normal HW drivers, so this patch moves the module
reference from the transport class functions shared by all
drivers to the libiscsi functions only used be software iscsi
modules.

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