bridge: avoid br_ifinfo_notify when nothing changed

When neither IFF_BRIDGE nor IFF_BRIDGE_PORT is set,
and afspec == NULL but  protinfo != NULL, we run into
"if (err == 0) br_ifinfo_notify(RTM_NEWLINK, p);" with
random value in ret.

Thanks to Sergei for pointing out the error in commit comments.

Signed-off-by: Hong Zhiguo <honkiko@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>