lib/cmap: Return number of nodes from functions modifying the cmap.

lib/cmap: Return number of nodes from functions modifying the cmap.

We already update the count field as the last step of these functions,
so returning the current count is very cheap.  Callers that care about
the count become a bit more efficient, as they avoid extra
non-inlineable function call.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>