[SCSI] aacraid: use kthread_ API

Use the kthread_ API instead of opencoding lots of hairy code for kernel
thread creation and teardown.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Salyzyn, Mark <mark_salyzyn@adaptec.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>