[SCSI] SCSI core: fix leakage of scsi_cmnd's

Actually, just one problem and one cosmetic fix:

1) We need to dequeue for the loop and kill case (it seems easiest
simply to dequeue in the scsi_kill_request() routine)
2) There's no real need to drop the queue lock.  __scsi_done() is lock
agnostic, so since there's no requirement, let's just leave it in to
avoid any locking issues.

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