Always allocate per-CPU statistics counters for network interface

Always allocate per-CPU statistics counters for network interface
descriptor.

We have the mess in network interface statistics. Only pseudo drivers
do per-CPU counters allocation, all other network devices use the old
`if_data'. The network stack partially uses per-CPU counters and
partially use `if_data', but the protection is inconsistent: some times
counters accessed with exclusive netlock, some times with shared
netlock, some times with kernel lock, but without netlock, some times
with another locks.

To make network interfaces statistics more consistent, always allocate
per-CPU counters at interface attachment time and use it instead of
`if_data'. At this step only move counters allocation to the if_attach()
internals. The `if_data' removal will be performed with the following
diffs to make review and tests easier.

ok bluhm