UPSTREAM: selinux: Use kmem_cache for hashtab_node

During random test as own device to check slub account,
we found some slack memory from hashtab_node(kmalloc-64).
By using kzalloc(), middle of test result like below:
allocated size 240768
request size 45144
slack size 195624
allocation count 3762

So, we want to use kmem_cache_zalloc() and that
can reduce memory size 52byte(slack size/alloc count) per each struct.

Signed-off-by: Kyeongdon Kim <kyeongdon.kim@lge.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

(cherry picked from commit 7c620ece125cbab7b5dfcb574ee1e64ab8b562cd)
Change-Id: I552ed33d1d51f6c0fc37d078ed8f5cf07e9d76aa
Bug: 140252993
Signed-off-by: Jeff Vander Stoep <jeffv@google.com>