acl-plugin: multicore: use pool_init_fixed for per-worker preallocated pools

One worker thread may be processing the packets
for session owned by another worker thread.
During session access the validity of the pool
index is checked - however, the free bitmap pointer
might change just at that moment, potentially resulting
in a crash.

Thus, use the pool_init_fixed() when initializing the per-worker pools,
so that the free bitmaps are as well staying in their place.

Change-Id: I5796e6b62fdc1efd4299124a388b84a7c0dc19cd
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>