SUNRPC: add 'struct cred *' to auth_cred and rpc_cred

The SUNRPC credential framework was put together before
Linux has 'struct cred'.  Now that we have it, it makes sense to
use it.
This first step just includes a suitable 'struct cred *' pointer
in every 'struct auth_cred' and almost every 'struct rpc_cred'.

The rpc_cred used for auth_null has a NULL 'struct cred *' as nothing
else really makes sense.

For rpc_cred, the pointer is reference counted.
For auth_cred it isn't.  struct auth_cred are either allocated on
the stack, in which case the thread owns a reference to the auth,
or are part of 'struct generic_cred' in which case gc_base owns the
reference, and "acred" shares it.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/auth_unix.c

@@ -97,6 +97,7 @@ static void
 unx_free_cred(struct unx_cred *unx_cred)
 {
 	dprintk("RPC:       unx_free_cred %p\n", unx_cred);
+	put_cred(unx_cred->uc_base.cr_cred);
 	kfree(unx_cred);
 }