dpif-netdev: Fix unsafe access to pmd polling lists.

All accesses to 'pmd->poll_list' should be guarded by
'pmd->port_mutex'. Additionally fixed inappropriate usage
of 'HMAP_FOR_EACH_SAFE' (hmap doesn't change in a loop)
and dropped not needed local variable 'proc_cycles'.

CC: Nitin Katiyar <nitin.katiyar@ericsson.com>
Fixes: 5bf8428 ("Adding support for PMD auto load balancing")
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Kevin Traynor <ktraynor@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>

lib/dpif-netdev.c

@@ -5098,7 +5098,7 @@ pmd_rebalance_dry_run(struct dp_netdev *dp)
     uint64_t improvement = 0;
     uint32_t num_pmds;
     uint32_t *pmd_corelist;
-    struct rxq_poll *poll, *poll_next;
+    struct rxq_poll *poll;
     bool ret;
 
     num_pmds = cmap_count(&dp->poll_threads);
@@ -5124,13 +5124,14 @@ pmd_rebalance_dry_run(struct dp_netdev *dp)
         /* Estimate the cycles to cover all intervals. */
         total_cycles *= PMD_RXQ_INTERVAL_MAX;
 
-        HMAP_FOR_EACH_SAFE (poll, poll_next, node, &pmd->poll_list) {
-            uint64_t proc_cycles = 0;
+        ovs_mutex_lock(&pmd->port_mutex);
+        HMAP_FOR_EACH (poll, node, &pmd->poll_list) {
             for (unsigned i = 0; i < PMD_RXQ_INTERVAL_MAX; i++) {
-                proc_cycles += dp_netdev_rxq_get_intrvl_cycles(poll->rxq, i);
+                total_proc += dp_netdev_rxq_get_intrvl_cycles(poll->rxq, i);
             }
-            total_proc += proc_cycles;
         }
+        ovs_mutex_unlock(&pmd->port_mutex);
+
         if (total_proc) {
             curr_pmd_usage[num_pmds] = (total_proc * 100) / total_cycles;
         }