veth: fix 64bit stats on 32bit arches

Using 64bit stats on 32bit arches must use a synchronization or readers
can get transient values.

Fixes bug introduced in commit 6311cc44 (veth: convert to 64 bit
statistics)

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>