loci: fix packet out action len for OF 1.1+

OF 1.1+ packet-out messages have 6 bytes of padding in the fixed length part of
the object after actions_len.
loci: fix of_wire_buffer_replace_data for non-trailing members

The previous code only worked correctly if the member being grown was the last
non-zero-length member in the object.
loci: fix OF 1.2+ packet-in data offset

There are two bytes of padding between the match and the packet data.

This commit introduces "extra length", which is the sum of the fixed length
parts of an object having variable offsets (after a variable length member).
The only use is for OF 1.2+ packet-in messages.