openvswitch: Relax set header validation.

When installing a flow with an action to set a particular field we
need to validate that the packets that are part of the flow actually
contain that header.  With IP we use zeroed addresses and with TCP/UDP
the check is for zeroed ports.  This check is overly broad and can catch
packets like DHCP requests that have a zero source address in a
legitimate header.  This changes the check to look for a zeroed protocol
number for IP or for both ports be zero for TCP/UDP before considering
the header to not exist.

Reported-by: Ethan Jackson <ethan@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>