net: Validate IFLA_BRIDGE_MODE attribute length

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

Fixes: a77dcb8c ("be2net: set and query VEB/VEPA mode of the PF interface")
Fixes: 815cccbf

 ("ixgbe: add setlink, getlink support to ixgbe and ixgbevf")
Cc: Ajit Khaparde <ajit.khaparde@emulex.com>
Cc: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Acked-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>