sunrpc: add hash_cred() function to rpc_authops struct

Currently, a single hash algorithm is used to hash the auth_cred for
the credcache for all rpc_auth types.  Add a hash_cred() function to
the rpc_authops struct to allow a hash function specific to each
auth flavor.

Signed-off-by: Frank Sorenson <sorenson@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>