BACKPORT: Smack: Improve access check performance

BACKPORT: Smack: Improve access check performance

Each Smack label that the kernel has seen is added to a
list of labels. The list of access rules for a given subject
label hangs off of the label list entry for the label.
This patch changes the structures that contain subject
labels to point at the label list entry rather that the
label itself. Doing so removes a label list lookup in
smk_access() that was accounting for the largest single
chunk of Smack overhead.

Targeted for git://git.gitorious.org/smack-next/kernel.git

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
(cherry-picked from upstream 2f823ff8bec03a1e6f9e11fd0c4d54e4c7d09532)