net: bridge: Get SIOCGIFBR/SIOCSIFBR ioctl working in compat mode

In compat mode SIOC{G,S}IFBR ioctls were only supporting
BRCTL_GET_VERSION returning an artificially version to spur userland
tool to use SIOCDEVPRIVATE instead. But some userland tools ignore that
and use SIOC{G,S}IFBR unconditionally as seen with busybox's brctl.

Example of non working 32-bit brctl with CONFIG_COMPAT=y:
$ brctl show
brctl: SIOCGIFBR: Invalid argument

Example of fixed 32-bit brctl with CONFIG_COMPAT=y:
$ brctl show
bridge name     bridge id               STP enabled     interfaces
br0

Signed-off-by: Remi Pommarel <repk@triplefau.lt>
Co-developed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>