kernel - dm - Reorganize the crypt code and implement B_HASBOGUS

kernel - dm - Reorganize the crypt code and implement B_HASBOGUS

* Implement B_HASBOGUS data copying for read decryption.  When a buffer
  contains bogus pages we may or may not be able to safely decrypt data
  in-place, add some code to deal with it.

* Reorganize the crypt code.  Separate the read and write paths and
  organize the procedures in the same order as the execution stages.
  Also move some of the buffer manipulation around a little to make
  it more clear.

Submitted-by: alexh (B_HASBOGUS handling)