Use initializers for struct ofputil_flow_mod instead of assignments.

Use initializers for struct ofputil_flow_mod instead of assignments.

A few bugs have been fixed lately that were related to struct
ofputil_flow_mod not being fully initialized in a few places.  This commit
changes several pieces of code from using individual assignments to fields
in struct ofputil_flow_mod, to using whole initializers or assignments to
a whole struct.  This should help prevent similar problems in the future.

CC: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Russell Bryant <russell@ovn.org>