tipc: convert legacy nl stats show to nl compat

Convert TIPC_CMD_SHOW_STATS to compat layer. This command does not
have any counterpart in the new API, meaning it now solely exists as a
function in the compat layer.

Signed-off-by: Richard Alpe <richard.alpe@ericsson.com>
Reviewed-by: Erik Hugne <erik.hugne@ericsson.com>
Reviewed-by: Ying Xue <ying.xue@windriver.com>
Reviewed-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>