Rearrange structures to better fit valgrind's memory leak heuristics.

Rearrange structures to better fit valgrind's memory leak heuristics.

valgrind's memory leak detector considers a pointer to the head of a memory
block to be "definitely" a pointer to that memory block but a pointer to
the interior of a memory block only "possibly" a pointer to that memory
block.  Open vSwitch hmap_node and list data structures can go anywhere
inside a structure; if they are in the middle of a structure then valgrind
considers pointers to them to be possible leaks.  Therefore, this commit
moves some of these from the middle of data structures to the head, to
reduce valgrind's uncertainty.

Signed-off-by: Ben Pfaff <blp@nicira.com>