Fix setting transport ports with frags.

Fix setting transport ports with frags.

Packets with 'LATER' fragment do not have a transport header, so it is
not possible to either match on or set transport ports on such
packets.  Matching is prevented by augmenting mf_are_prereqs_ok() with
a nw_frag 'LATER' bit check.  Setting the transport headers on such
packets is prevented in three ways:

1. Flows with an explicit match on nw_frag, where the LATER bit is 1:
   existing calls to the modified mf_are_prereqs_ok() prohibit using
   transport header fields (port numbers) in OXM/NXM actions
   (set_field, move).  SET_TP_* actions need a new check on the LATER
   bit.

2. Flows that wildcard the nw_frag LATER bit: At flow translation
   time, add calls to mf_are_prereqs_ok() to make sure that we do not
   use transport ports in flows that do not have them.

3. At action execution time, do not set transport ports, if the packet
   does not have a full transport header.  This ensures that we never
   call the packet_set functions, that require a valid transport
   header, with packets that do not have them.  For example, if the
   flow was created with a IPv6 first fragment that had the full TCP
   header, but the next packet's first fragment is missing them.

3 alone would suffice for correct behavior, but 1 and 2 seem like a
right thing to do, anyway.

Currently, if we are setting port numbers, we will also match them,
due to us tracking the set fields with the same flow_wildcards as the
matched fields.  Hence, if the incoming port number was not zero, the
flow would not match any packets with missing or truncated transport
headers.  However, relying on no packets having zero port numbers
would not be very robust.  Also, we may separate the tracking of set
and matched fields in the future, which would allow some flows that
blindly set port numbers to not match on them at all.

For TCP in case 3 we use ofpbuf_get_tcp_payload() that requires the
whole (potentially variable size) TCP header to be present.  However,
when parsing a flow, we only require the fixed size portion of the TCP
header to be present, which would be enough to set the port numbers
and fix the TCP checksum.

Finally, we add tests testing the new behavior.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>