netdev_alloc_pcpu_stats: use less common iterator variable

With the CPU iteration variable called 'i', it's relatively easy
to have variable shadowing which sparse will warn about. Avoid
that by renaming the variable to __cpu which is less likely to
be used in the surrounding context.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>