net: prevent sign extension in dev_get_stats()

Similar to the fix provided by Dominik Heidler in commit
9b3dc0a1 ("l2tp: cast l2tp traffic counter to unsigned")
we need to take care of 32bit kernels in dev_get_stats().

When using atomic_long_read(), we add a 'long' to u64 and
might misinterpret high order bit, unless we cast to unsigned.

Fixes: caf586e5 ("net: add a core netdev->rx_dropped counter")
Fixes: 015f0688 ("net: net: add a core netdev->tx_dropped counter")
Fixes: 6e7333d3

 ("net: add rx_nohandler stat counter")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>