openvswitch: Properly set L4 keys on "later" IP fragments

When IP fragments are reassembled before being sent to conntrack, the
key from the last fragment is used.  Unless there are reordering
issues, the last fragment received will not contain the L4 ports, so the
key for the reassembled datagram won't contain them.  This patch updates
the key once we have a reassembled datagram.

The handle_fragments() function works on L3 headers so we pull the L3/L4
flow key update code from key_extract into a new function
'key_extract_l3l4'.  Then we add a another new function
ovs_flow_key_update_l3l4() and export it so that it is accessible by
handle_fragments() for conntrack packet reassembly.

Co-authored-by: Justin Pettit <jpettit@ovn.org>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>