flow: Fix struct flow size build assertion failure.

flow: Fix struct flow size build assertion failure.

The new struct flow_tnl contains an extra four bytes of padding on
64-bit machines but we currently assert that the total struct flow
is a fixed size.  The size difference isn't actually a problem
because both are multiples of 4 and the build assertion is only
intended to remind people to update FLOW_WC_SEQ when new fields are
added.  This changes the assertion to fix just the non-tunnel field
size.

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