selinux: wrap AVC state

Wrap the AVC state within the selinux_state structure and
pass it explicitly to all AVC functions.  The AVC private state
is encapsulated in a selinux_avc structure that is referenced
from the selinux_state.

This change should have no effect on SELinux behavior or
APIs (userspace or LSM).

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Reviewed-by: James Morris <james.morris@microsoft.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>