gc_flavor is used only for looking up the security service, which is an
 integer value that never changes. Store the latter instead of the former.

 Fix up a couple of dodgy casts between gss_cred and rpc_cred. Replace them
 with the appropriate container_of().
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>