tipc: add broadcast link window set/get to nl api

Add the ability to get or set the broadcast link window through the
new netlink API. The functionality was unintentionally missing from
the new netlink API. Adding this means that we also fix the breakage
in the old API when coming through the compat layer.

Fixes: 37e2d484

 (tipc: convert legacy nl link prop set to nl compat)
Reported-by: Tomi Ollila <tomi.ollila@iki.fi>
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>
Signed-off-by: David S. Miller <davem@davemloft.net>