dm_target_crypt - rework ivgen magic, fix bug

dm_target_crypt - rework ivgen magic, fix bug

* Add a forgotten crypto_freesession() in the destruction of the target.

* Change ivgen infrastructure to be more complete; each ivgen can now
  have a ctor, a dtor and some private data.

* Change ESSIV ivgen to use its own crypto session.

* Change all the crypto stuff *not* to use an explicit key but rather
  use the key with which they were initialized.

* Dispatch the actual data crypto from the ivgens, allowing the iv
  generation to happen asynchronously.

Discussed-with: Matt Dillon (dillon@)