Create crypt_iv_operations structure with generator method and move the
plain iv generator into this structure.  Optionally accept an extended
<sector format> syntax: <cipher>-<chaining mode>-<iv mode>

This also makes it ready to support chaining modes other than CBC 
mode, such as CMC (not implemented in cryptoapi yet), 
The problems outlined by Adam J. Richter in
  http://article.gmane.org/gmane.linux.kernel.device-mapper.dm-crypt/454
would be fixed by switching to CMC chaining mode.

Example of a valid target line:
  0 200 crypt aes-cbc-plain 00000000000000000000000000000000 0 /dev/loop/5 0
Signed-Off-By: Alasdair G Kergon <agk@redhat.com>
Signed-Off-By: Christophe Saout <christophe@saout.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>