net: bridge: export supported boolopts

Now that we have at least one bool option, we can export all of the
supported bool options via optmask when dumping them.

v2: new patch

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>