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

Also switch from semaphore-based thread wakeup to wake_up_process.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Acked-By: NAndrew Vasquez <andrew.vasquez@qlogic.com>
Signed-off-by: NJames Bottomley <James.Bottomley@SteelEye.com>