l2tp: cast l2tp traffic counter to unsigned

l2tp: cast l2tp traffic counter to unsigned

This fixes a counter problem on 32bit systems:
When the rx_bytes counter reached 2 GiB, it jumpd to (2^64 Bytes - 2GiB) Bytes.

rtnl_link_stats64 has __u64 type and atomic_long_read returns
atomic_long_t which is signed. Due to the conversation
we get an incorrect value on 32bit systems if the MSB of
the atomic_long_t value is set.

CC: Tom Parkin <[email protected]>
Fixes: 7b7c0719cd7a ("l2tp: avoid deadlock in l2tp stats update")
Signed-off-by: Dominik Heidler <[email protected]>
Signed-off-by: David S. Miller <[email protected]>