datapath: Better calculate max nlattr-formatted flow size.

datapath: Better calculate max nlattr-formatted flow size.

Both userspace and the kernel allocate space based on the max size of a
nlattr-formatted flow.  It was easy to change the max size of a flow
definition and cause crashes by forgetting to update one or both of
those definitions.  This commit attempts to make that harder by
providing a better description of how the max size is calculated and a
build check to look for a common indication that it may have changed.

Signed-off-by: Justin Pettit <jpettit@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>