net-sysfs: add backlog len and CPU id to softnet data

Currently the backlog status in not exposed to user-space.
Since long backlogs (or simply not empty ones) can be a
source of noticeable latency, -RT deployments need some way
to inspect it.

Additionally, there isn't a direct match between 'softnet_stat'
lines and the related CPU - sd for offline CPUs are not dumped -
so this patch also includes the CPU id into such entry.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>