Fix variable uninitialisation warnings introduced in:

	commit 8bbf4976
	Author: David Howells <dhowells@redhat.com>
	Date:   Fri Nov 14 10:39:14 2008 +1100

	KEYS: Alter use of key instantiation link-to-keyring argument

As:

  security/keys/keyctl.c: In function 'keyctl_negate_key':
  security/keys/keyctl.c:976: warning: 'dest_keyring' may be used uninitialized in this function
  security/keys/keyctl.c: In function 'keyctl_instantiate_key':
  security/keys/keyctl.c:898: warning: 'dest_keyring' may be used uninitialized in this function

Some versions of gcc notice that get_instantiation_key() doesn't always set
*_dest_keyring, but fail to observe that if this happens then *_dest_keyring
will not be read by the caller.
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>