[NET_SCHED]: cls_fw: fix NULL pointer dereference

When the first fw classifier is initialized, there is a small window
between the ->init() and ->change() calls, during which the classifier
is active but not entirely set up and tp->root is still NULL (->init()
does nothing).

When a packet is queued during this window a NULL pointer dereference
occurs in fw_classify() when trying to dereference head->mask;

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>