Refactor softraid crypto code to allow use of a discipline-specific data

Refactor softraid crypto code to allow use of a discipline-specific data
structure for RAID1C volumes, as requested by jsing@ during review of my
initial RAID1C patch.

This should effectively be a cosmetic change. The point of this exercise
is to allow the data structure changes made here in softraidvar.h.

Tested on CRYPTO and RAID1C by Josh Grosse, Matthias Schmidt, and myself.
ok jsing@