net: fix bridge notify hook to manage flags correctly

The bridge notify hook rtnl_bridge_notify() was not handling the
case where the master flags was set or with both flags set. First
flags are not being passed correctly and second the logic to parse
them is broken.

This patch passes the original flags value and fixes the
logic.

Reported-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>