cmap: Fix cmap_next_position()

cmap_next_position() didn't update the node pointer while iterating through a
list of nodes with the same hash.
This commit fixes the bug and improve test-cmap to detect it.

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>