Make security document refs valid.
Signed-off-by: NTom Saeger <tom.saeger@oracle.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

The existing LSM.txt file covered both usage and development, so split
this into two files, one under admin-guide and one under kernel
development.

Cc: James Morris <james.l.morris@oracle.com>
Signed-off-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>