nx-match: Separate raw match and header/pad pull/put

nx-match: Separate raw match and header/pad pull/put

In the case of Open Flow 1.2, which is currently the only
time that OXM is be used, there is a 4 byte header before
the match which needs to be taken into account when calculating
the pad length. This complicates nx_match pull and put somewhat.

This patch takes an approach suggested by Ben Pfaff to separate the
encoding of the match and the adding of padding and, in the case of OXM,
a header.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>