rpcgss: simplify make_checksum

We're doing some pointless translation between krb5 constants and kernel
crypto string names.

Also clean up some related spkm3 code as necessary.

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