No need to make_dev() existing root device in disk crypto question

No need to make_dev() existing root device in disk crypto question

Since r1.1245 encrypt_root() happens immediately after get_rootinfo().
the latter creates device files for the root disk (and aborts if make_dev()
fails), so encrypt_root()'s call on the softraid chunk is purely redundant.

Hoist _chunk definition into declaration while here.