cciss: factor out scatter gather chain block allocation and freeing

cciss: factor out scatter gather chain block allocation and freeing
Rationale is that I want to use this code from the scsi half of the
driver.

Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>