[SCSI] use scatter lists for all block pc requests and simplify hw handlers

Original From: Mike Christie <michaelc@cs.wisc.edu>

Add scsi_execute_req() as a replacement for scsi_wait_req()

Fixed up various pieces (added REQ_SPECIAL and caught req use after
free)

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>