SELinux: convert the avc cache hash list to an hlist

We do not need O(1) access to the tail of the avc cache lists and so we are
wasting lots of space using struct list_head instead of struct hlist_head.
This patch converts the avc cache to use hlists in which there is a single
pointer from the head which saves us about 4k of global memory.

Resulted in about a 1.5% decrease in time spent in avc_has_perm_noaudit based
on oprofile sampling of tbench.  Although likely within the noise....

Signed-off-by: Eric Paris <eparis@redhat.com>
Reviewed-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@namei.org>