LSM: Add the comment to lsm_hooks.h

Add the large comment describing the content of the
security_operations structure to lsm_hooks.h. This
wasn't done in the previous (1/7) patch because it
would have exceeded the mail list size limits.
Signed-off-by: NCasey Schaufler <casey@schaufler-ca.com>
Acked-by: NJohn Johansen <john.johansen@canonical.com>
Acked-by: NKees Cook <keescook@chromium.org>
Acked-by: NPaul Moore <paul@paul-moore.com>
Acked-by: NStephen Smalley <sds@tycho.nsa.gov>
Acked-by: NTetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: NJames Morris <james.l.morris@oracle.com>