Set ide_task_t fields (command_type, handler and prehandler) directly.
Remove unused ide_task_t->posthandler and all ide_cmd_type_parser() logic.

ide_cmd_type_parser() was meant to be used for ioctls but
ended up checking validity of kernel generated requests (doh!).

Rationale for removal:
- it can't be used for existing ioctls (changes the way they work)
- kernel shouldn't check validity of (root only) user-space requests
  (it can and should be done in user-space)
- it wastes CPU cycles on going through parsers
- it makes code harder to understand/follow
  (now info about request is localized)