cifscreds: make cifscreds use the "logon" key_type

cifscreds: make cifscreds use the "logon" key_type

...and have it loosen the permissions to allow searching. There seems
to be no clear way to make user keys unreadable, but still allow for
them to be searched, so we'll need a new key_type that doesn't allow
you to read the payload from userspace. That will be proposed in a
separate kernel patch.

Signed-off-by: Jeff Layton <jlayton@samba.org>