bridge: Validate IFLA_BRIDGE_FLAGS attribute length

Payload is currently accessed blindly and may exceed valid message
boundaries.

Fixes: 407af329

 ("bridge: Add netlink interface to configure vlans on bridge ports")
Cc: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>