1. Add support for Task collector mode.
2. Fixed relative collector mode bug:
   - I/O failed when disks is on two ports
   - system hang when hotplug disk
   - system hang when unplug disk during run IO
3. Unlock ap->lock within .lldd_execute_task for direct mode to
   improve performance
Signed-off-by: Xiangliang Yu <yuxiangl@marvell.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>