flow: Pass last field to miniflow_pad_to_64().

flow: Pass last field to miniflow_pad_to_64().

Make miniflow_pad_to_64() a little more robust with regards to updates to
struct flow by passing the last field, whose end should be considered for
padding, rather than the next field, whose start should be considered.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>