CRED: Fix prepare_kernel_cred() to provide a new thread_group_cred struct

Fix prepare_kernel_cred() to provide a new, separate thread_group_cred struct
otherwise when using request_key() ____call_usermodehelper() calls
umh_keys_init() with the new creds pointing to init_tgcred, which
umh_keys_init() then blithely alters.

The problem can be demonstrated by:

	# keyctl request2 user a debug:a @s
	249681132
	# grep req /proc/keys
	079906a5 I--Q--     1 perm 1f3f0000     0     0 keyring   _req.249681132: 1/4
	38ef1626 IR----     1 expd 0b010000     0     0 .request_ key:ee1d4ec pid:4371 ci:1

The keyring _req.XXXX should have gone away, but something (init_tgcred) is
pinning it.

That key actually requested can then be removed and a new one created:

	# keyctl unlink 249681132
	1 links removed
	[root@andromeda ~]# grep req /proc/keys
	116cecac IR----     1 expd 0b010000     0     0 .request_ key:eeb4911 pid:4379 ci:1
	36d1cbf8 I--Q--     1 perm 1f3f0000     0     0 keyring   _req.250300689: 1/4

which causes the old _req keyring to go away and a new one to take its place.

This is a consequence of the changes in:

	commit 87966996
	Author: David Howells <dhowells@redhat.com>
	Date:   Fri Jun 17 11:25:59 2011 +0100
	KEYS/DNS: Fix ____call_usermodehelper() to not lose the session keyring

and:

	commit 17f60a7d
	Author: Eric Paris <eparis@redhat.com>
	Date:   Fri Apr 1 17:07:50 2011 -0400
	capabilites: allow the application of capability limits to usermode helpers

After this patch is applied, the _req keyring and the .request_key key are
cleaned up.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>