datapath: Properly initialize ovs_skb_cb of packet from userspace.

datapath: Properly initialize ovs_skb_cb of packet from userspace.

The ovs_skb_cb in 'packet' in this function is initially a clone of the
corresponding area in 'skb', which came from the Netlink layer and thus
isn't necessarily all-zeros.  This commit initializes it properly before
passing it along to execute_actions().

The most common problem caused by failing to initialize the ovs_skb_cb
properly was that on Linux 2.6.26 and earlier, where Open vSwitch keeps
its own vlan_tci field inside ovs_skb_cb, the first packet of a flow would
get sent out tagged with a random VLAN (usually 0x0001 or 0xffff in our
testing).  This commit should fix that problem.

Another likely problem would be for turning on sFlow to randomly panic the
kernel.  That problem would not be kernel version dependent.  We haven't
been testing sFlow so we haven't noticed this problem.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>
Reported-by: Pankaj Thakkar <thakkar@nicira.com>