selinux: Introduce a new config to make avc cache slot size adjustable

On mobile device high-load situations, permission check can happen
more than 90,000/s (8 core system). With default 512 cache nodes
configuration, avc cache miss happens more often and occasionally
leads to long time (>2ms) irqs off on both big and little cores,
which decreases system real-time capability.

An actual call stack is as follows:
 => avc_compute_av
 => avc_perm_nonode
 => avc_has_perm_noaudit
 => selinux_capable
 => security_capable
 => capable
 => __sched_setscheduler
 => do_sched_setscheduler
 => __arm64_sys_sched_setscheduler
 => invoke_syscall
 => el0_svc_common
 => do_el0_svc
 => el0_svc
 => el0t_64_sync_handler
 => el0t_64_sync

Although we can expand avc nodes through /sys/fs/selinux/cache_threshold
to mitigate long time irqs off, hash conflicts make the bucket average
length longer because of the fixed size of cache slots, leading to
avc_search_node() latency increase.

So introduce a new config to make avc cache slot size also configurable,
and with fine tuning, we can mitigate long time irqs off with slightly
avc_search_node() performance regression.

Theoretically, the main overhead is memory consumption.

Signed-off-by: Hongru Zhang <zhanghongru@xiaomi.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>