nx-match: Enable senders of NXM fields to specify length.

nx-match: Enable senders of NXM fields to specify length.

Currently when an NXM field is encoded, the caller must specify
the length of the data being provided. However, this data is
always placed into a field of standard length. In order to
support variable length options, the length field must also
alter the size in the header. The previous implementation
already required callers to pass in the exact (fixed) size of
the field or it would not work properly, so there is no danger
that this will change the behavior for non-variable length
fields.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>