flow: Get rid of flow_t typedef.

flow: Get rid of flow_t typedef.

When userspace and the kernel were using the same structure for flows,
flow_t was a useful way to indicate that a structure was really a userspace
flow instead of a kernel one, but now it's better to just write "struct
flow" for consistency, since OVS doesn't use typedefs for structs
elsewhere.

Acked-by: Jesse Gross <jesse@nicira.com>