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

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: 5bf84282482a ("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>