dm crypt: remove obsolete references to per-CPU state

dm-crypt used to use separate crypto transforms for each CPU, but this
is no longer the case.  To avoid confusion, fix up obsolete comments and
rename setup_essiv_cpu().

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>