This changes security_hook_heads to use hlist_heads instead of
the circular doubly-linked list heads. This should cut down
the size of the struct by about half.

In addition, it allows mutation of the hooks at the tail of the
callback list without having to modify the head. The longer-term
purpose of this is to enable making the heads read only.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Reviewed-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: James Morris <james.morris@microsoft.com>