At one time, the ofproto-dpif implementation of the 'rule_execute' member
function required, indirectly, at least struct(ofp10_packet_in) bytes of
headroom in the packet passed into it. (This allowed constructing an
OFPT_PACKET_IN without allocating and copying a new ofpbuf.) This
restriction has long been lifted, but rule_execute() had not yet caught
up. This commit updates it.