device mapper - implement dumping

device mapper - implement dumping

* Implement dmdump and dump routines for the three main targets (linear,
  stripe and crypt).

* The top-level dmpdump will call all the required dump() methods in the
  targets just as it does with strategy() calls. The lower level
  target-specific dump routines will then redirect (after processing,
  etc) these requests to the underlying device's dump routines.

* This should provide quite reliable dumping even through device mapper,
  although it is more error-prone than the equivalent dumping on normal
  disks as there's a lot more going on behind the scenes.