flow: free spare pool more aggressively

The flows exceeding the spare pools config setting would be freed
per at max 100 flows a second. After a high speed test this would
lead to excessive memory use for a long time.

This patch updates the logic to free 10% of the excess flows per
run, freeing multiple blocks of flows as needed.

Bug: #4731.

src/flow-spare-pool.c

@@ -173,8 +173,8 @@ void FlowSparePoolUpdate(uint32_t size)
 {
     const int64_t todo = (int64_t)flow_config.prealloc - (int64_t)size;
     if (todo < 0) {
-        /* remove one block at most at a time */
         uint32_t to_remove = (uint32_t)(todo * -1) / 10;
+        while (to_remove) {
             if (to_remove < flow_spare_pool_block_size)
                 return;
 
@@ -184,6 +184,7 @@ void FlowSparePoolUpdate(uint32_t size)
             if (p != NULL) {
                 flow_spare_pool = p->next;
                 flow_spare_pool_flow_cnt -= p->queue.len;
+                to_remove -= p->queue.len;
             }
             SCMutexUnlock(&flow_spare_pool_m);
 
@@ -194,7 +195,7 @@ void FlowSparePoolUpdate(uint32_t size)
                 }
                 SCFree(p);
             }
-
+        }
     } else if (todo > 0) {
         FlowSparePool *head = NULL, *tail = NULL;