security/keys: export key_lookup()

For in-kernel consumers one cannot readily assign a user (eg when
running from a workqueue), so the normal key search permissions
cannot be applied.
This patch exports the 'key_lookup()' function for a simple lookup
of keys without checking for permissions.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>