scsi: iscsi: Move pool freeing

scsi: iscsi: Move pool freeing

This doesn't fix any bugs, but it makes more sense to free the pool after
we have removed the session. At that time we know nothing is touching any
of the session fields, because all devices have been removed and scans are
stopped.

Link: https://lore.kernel.org/r/20210525181821.7617-19-michael.christie@oracle.com
Reviewed-by: Lee Duncan <lduncan@suse.com>
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>