flow: Separate "flow_t" from "struct odp_flow_key".

flow: Separate "flow_t" from "struct odp_flow_key".

The "struct odp_flow_key" used in the kernel datapath is conceptually
separate from the "flow_t" used in userspace, but until now we have
used the latter as a typedef for the former for convenience.  This commit
separates them.  This makes it possible in upcoming commits to change
them independently.