spkm3: fix spkm3's use of hmac

I think I botched an attempt to keep an spkm3 patch up-to-date with a recent
crypto api change.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>